Doctest example for eof (#46012)

Author: kshyatt

base/io.jl

@@ -173,6 +173,19 @@ function will block to wait for more data if necessary, and then return `false`.
 it is always safe to read one byte after seeing `eof` return `false`. `eof` will return
 `false` as long as buffered data is still available, even if the remote end of a connection
 is closed.
+
+# Examples
+```jldoctest
+julia> b = IOBuffer("my buffer");
+
+julia> eof(b)
+false
+
+julia> seekend(b);
+
+julia> eof(b)
+true
+```
 """
 function eof end