Expand help text for go mod tidy

`go mod tidy` will ignore files with the build constraint `ignore`. This
is documented in https://go.dev/ref/mod#go-mod-tidy. While this URL in
mentioned in the existing help text, the fact about the special handling
of the `ignore` tag is significant enough to be mentioned explicitly in
the help text. From the existing link it's not inmediately obvious that
this special behavior exists.

Take most of the relevant text from the online description and add it to
the help text of `go mod tidy`.

Fixes: golang#54993
Signed-off-by: Marcelo E. Magallon <[email protected]>

src/cmd/go/internal/modcmd/tidy.go

@@ -48,6 +48,15 @@ version. By default, tidy acts as if the -compat flag were set to the
 version prior to the one indicated by the 'go' directive in the go.mod
 file.
 
+go mod tidy works by loading all of the packages in the main module
+and all of the packages they import, recursively. This includes
+packages imported by tests (including tests in other modules). go mod
+tidy acts as if all build tags are enabled, so it will consider
+platform-specific source files and files that require custom build
+tags, even if those source files wouldn’t normally be built. There is
+one exception: the ignore build tag is NOT enabled, so a file with the
+build constraint //go:build ignore will not be considered.
+
 See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 	`,
 	Run: runTidy,