cmd/go: document GO{NO}SUMDB in 'go help environment' and 'go help modules'

GOSUMDB and GONOSUMDB are described in detail by
'go help module-auth'. So, this change mentions the two
variables and says to see 'go help module-auth'.

This also adds GONOPROXY to 'go help environment'.

Fixes #32292 and updates #32056.

Change-Id: I2eae0f906a3fbfcb88ad5c8fb4870917d0d7bbcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/179219
Reviewed-by: Russ Cox <[email protected]>

Author: tbpg

src/cmd/go/alldocs.go

@@ -509,7 +509,18 @@ General-purpose environment variables:
 	GOPATH
 		For more details see: 'go help gopath'.
 	GOPROXY
-		URL of Go module proxy. See 'go help goproxy'.
+		URL of Go module proxy. See 'go help modules'.
+	GONOPROXY
+		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
+		of module path prefixes that should always be fetched directly, ignoring
+		the GOPROXY setting. See 'go help modules'.
+	GOSUMDB
+		The name of checksum database to use and optionally its public key and
+		URL. See 'go help module-auth'.
+	GONOSUMDB
+		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
+		of module path prefixes that should not be compared against the checksum
+		database. See 'go help module-auth'.
 	GOROOT
 		The root of the go tree.
 	GOTMPDIR

src/cmd/go/internal/help/helpdoc.go

@@ -365,8 +365,9 @@ for future go command invocations.
 No matter the source of the modules, the go command checks downloads against
 known checksums, to detect unexpected changes in the content of any specific
 module version from one day to the next. This check first consults the current
-module's go.sum file but falls back to the Go checksum database.
-See 'go help module-auth' for details.
+module's go.sum file but falls back to the Go checksum database, controlled by
+the GOSUMDB and GONOSUMDB environment variables. See 'go help module-auth'
+for details.
 
 See 'go help goproxy' for details about the proxy protocol and also
 the format of the cached downloaded packages.