Add target.<cfg>.linker document

src/doc/src/reference/config.md

@@ -1098,6 +1098,17 @@ This option is deprecated and unused.
 Specifies the linker which is passed to `rustc` (via [`-C linker`]) when the
 [`<triple>`] is being compiled for. By default, the linker is not overridden.
 
+##### `target.<cfg>.linker`
+This is similar to the [target linker](#targettriplelinker), but using
+a [`cfg()` expression]. 
+
+The config precedence is:
+1. `host.<triple>.linker`, if [host-config](unstable.md#host-config) is enabled.
+2. `target.<triple>.linker`
+3. `target.<cfg>.linker`
+
+It is an error if more than one `<cfg>` runner matches the current target.
+
 ##### `target.<triple>.runner`
 * Type: string or array of strings ([program path with args])
 * Default: none