cmd/go: document default GOARM value

For #24904
For #58884

Change-Id: I0fc61f1a0bc6ebb22bdfd7a7b13ef36c68879384
Reviewed-on: https://go-review.googlesource.com/c/go/+/639276
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Commit-Queue: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: ianlancetaylor

src/cmd/go/alldocs.go

@@ -620,6 +620,11 @@ Architecture-specific environment variables:
 	GOARM
 		For GOARCH=arm, the ARM architecture for which to compile.
 		Valid values are 5, 6, 7.
+		When the Go tools are built on an arm system,
+		the default value is set based on what the build system supports.
+		When the Go tools are not built on an arm system
+		(that is, when building a cross-compiler),
+		the default value is 7.
 		The value can be followed by an option specifying how to implement floating point instructions.
 		Valid options are ,softfloat (default for 5) and ,hardfloat (default for 6 and 7).
 	GOARM64