cmd/compile: document -trimpath

Fixes #8999.

Change-Id: I1390605bdf908f59b596975ea51eb04bd03bbae0
Reviewed-on: https://go-review.googlesource.com/16918Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/doc.go

@@ -79,6 +79,8 @@ Flags:
 		Write a package (archive) file rather than an object file
 	-race
 		Compile with race detector enabled.
+	-trimpath prefix
+		Remove prefix from recorded source file paths.
 	-u
 		Disallow importing packages not marked as safe; implies -nolocalimports.