cmd/go: document the -mod=mod option

Updates #33848

Change-Id: I505490906be7cd6fbcdc6a05c1017c779dbf7bba
Reviewed-on: https://go-review.googlesource.com/c/go/+/207397
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modload/help.go

@@ -179,6 +179,9 @@ If invoked with -mod=vendor, the go command assumes that the vendor
 directory holds the correct copies of dependencies and ignores
 the dependency descriptions in go.mod.
 
+If invoked with -mod=mod, the go command loads modules from the module cache
+even if there is a vendor directory present.
+
 Pseudo-versions
 
 The go.mod file and the go command more generally use semantic versions as

src/cmd/go/internal/work/build.go

@@ -100,7 +100,7 @@ and test commands:
 		build code that will be linked against shared libraries previously
 		created with -buildmode=shared.
 	-mod mode
-		module download mode to use: readonly or vendor.
+		module download mode to use: readonly, vendor, or mod.
 		See 'go help modules' for more.
 	-modcacherw
 		leave newly-created directories in the module cache read-write