cmd/go: document purego convention

Fixes golang#23172

Change-Id: I854e399471dfa22e62fbdec9719e561c4501e5ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/660136
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: seankhliao

src/cmd/go/alldocs.go

@@ -1009,6 +1009,12 @@ Naming a file dns_windows.go will cause it to be included only when
 building the package for Windows; similarly, math_386.s will be included
 only when building the package for 32-bit x86.
 
+By convention, packages with assembly implementations may provide a go-only
+version under the "purego" build constraint. This does not limit the use of
+cgo (use the "cgo" build constraint) or unsafe. For example:
+
+        //go:build purego
+
 Go versions 1.16 and earlier used a different syntax for build constraints,
 with a "// +build" prefix. The gofmt command will add an equivalent //go:build
 constraint when encountering the older syntax.