Rollup merge of rust-lang#56149 - ariasuni:improve-amctime-doc, r=TimNN

Make std::os::unix/linux::fs::MetadataExt::a/m/ctime* documentation clearer

I was confused by this API so I clarified what they are doing.

I was wondering if I should try to unify more documentation and examples between `unix` and `linux` (e.g. “of the file” is used in `unix` to refer to the file these metadata is for, “of this file” in `linux`, “of the underlying file” in `std::fs::File`).

src/libstd/os/linux/fs.rs

@@ -191,7 +191,7 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext2", since = "1.8.0")]
     fn st_size(&self) -> u64;
-    /// Returns the last access time.
+    /// Returns the last access time of the file, in seconds since Unix Epoch.
     ///
     /// # Examples
     ///
@@ -208,7 +208,9 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext2", since = "1.8.0")]
     fn st_atime(&self) -> i64;
-    /// Returns the last access time, nano seconds part.
+    /// Returns the last access time of the file, in nanoseconds since [`st_atime`].
+    ///
+    /// [`st_atime`]: #tymethod.st_atime
     ///
     /// # Examples
     ///
@@ -225,7 +227,7 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext2", since = "1.8.0")]
     fn st_atime_nsec(&self) -> i64;
-    /// Returns the last modification time.
+    /// Returns the last modification time of the file, in seconds since Unix Epoch.
     ///
     /// # Examples
     ///
@@ -242,7 +244,9 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext2", since = "1.8.0")]
     fn st_mtime(&self) -> i64;
-    /// Returns the last modification time, nano seconds part.
+    /// Returns the last modification time of the file, in nanoseconds since [`st_mtime`].
+    ///
+    /// [`st_mtime`]: #tymethod.st_mtime
     ///
     /// # Examples
     ///
@@ -259,7 +263,7 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext2", since = "1.8.0")]
     fn st_mtime_nsec(&self) -> i64;
-    /// Returns the last status change time.
+    /// Returns the last status change time of the file, in seconds since Unix Epoch.
     ///
     /// # Examples
     ///
@@ -276,7 +280,9 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext2", since = "1.8.0")]
     fn st_ctime(&self) -> i64;
-    /// Returns the last status change time, nano seconds part.
+    /// Returns the last status change time of the file, in nanoseconds since [`st_ctime`].
+    ///
+    /// [`st_ctime`]: #tymethod.st_ctime
     ///
     /// # Examples
     ///

src/libstd/sys/unix/ext/fs.rs

@@ -522,7 +522,7 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext", since = "1.1.0")]
     fn size(&self) -> u64;
-    /// Returns the time of the last access to the file.
+    /// Returns the last access time of the file, in seconds since Unix Epoch.
     ///
     /// # Examples
     ///
@@ -539,7 +539,9 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext", since = "1.1.0")]
     fn atime(&self) -> i64;
-    /// Returns the time of the last access to the file in nanoseconds.
+    /// Returns the last access time of the file, in nanoseconds since [`atime`].
+    ///
+    /// [`atime`]: #tymethod.atime
     ///
     /// # Examples
     ///
@@ -556,7 +558,7 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext", since = "1.1.0")]
     fn atime_nsec(&self) -> i64;
-    /// Returns the time of the last modification of the file.
+    /// Returns the last modification time of the file, in seconds since Unix Epoch.
     ///
     /// # Examples
     ///
@@ -573,7 +575,9 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext", since = "1.1.0")]
     fn mtime(&self) -> i64;
-    /// Returns the time of the last modification of the file in nanoseconds.
+    /// Returns the last modification time of the file, in nanoseconds since [`mtime`].
+    ///
+    /// [`mtime`]: #tymethod.mtime
     ///
     /// # Examples
     ///
@@ -590,7 +594,7 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext", since = "1.1.0")]
     fn mtime_nsec(&self) -> i64;
-    /// Returns the time of the last status change of the file.
+    /// Returns the last status change time of the file, in seconds since Unix Epoch.
     ///
     /// # Examples
     ///
@@ -607,7 +611,9 @@ pub trait MetadataExt {
     /// ```
     #[stable(feature = "metadata_ext", since = "1.1.0")]
     fn ctime(&self) -> i64;
-    /// Returns the time of the last status change of the file in nanoseconds.
+    /// Returns the last status change time of the file, in nanoseconds since [`ctime`].
+    ///
+    /// [`ctime`]: #tymethod.ctime
     ///
     /// # Examples
     ///