doc/go1.11, cmd/go: elaborate on new GOFLAGS environment variable

In Go 1.11, cmd/go gained support for the GOFLAGS environment variable.
It was added and described in detail in CL 126656.
Mention it in the Go 1.11 release notes, link to the cmd/go documentation,
and add more details there.

Fixes #27282.

Change-Id: Ifc35bfe3e0886a145478d36dde8e80aedd8ec68e
Reviewed-on: https://go-review.googlesource.com/135035Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>

doc/go1.11.html

@@ -348,6 +348,20 @@ updating. See the <a href="go1.10.html#cgo">Go 1.10 release notes</a> for
 details. <!-- CL 126275, CL 127156, CL 122217, CL 122575, CL 123177 -->
 </p>
 
+<h3 id="go_command">Go command</h3>
+
+<p><!-- CL 126656 -->
+  The environment variable <code>GOFLAGS</code> may now be used
+  to set default flags for the <code>go</code> command.
+  This is useful in certain situations.
+  Linking can be noticeably slower on underpowered systems due to DWARF,
+  and users may want to set <code>-ldflags=-w</code> by default.
+  For modules, some users and CI systems will want vendoring always,
+  so they should set <code>-mod=vendor</code> by default.
+  For more information, see the <a href="/cmd/go/#hdr-Environment_variables"><code>go</code>
+  command documentation</a>.
+</p>
+
 <h3 id="godoc">Godoc</h3>
 
 <p>

src/cmd/go/alldocs.go

@@ -1449,6 +1449,12 @@
 // 		The directory where the go command will write
 // 		temporary source files, packages, and binaries.
 //
+// Each entry in the GOFLAGS list must be a standalone flag.
+// Because the entries are space-separated, flag values must
+// not contain spaces. In some cases, you can provide multiple flag
+// values instead: for example, to set '-ldflags=-s -w'
+// you can use 'GOFLAGS=-ldflags=-s -ldflags=-w'.
+//
 // Environment variables for use with cgo:
 //
 // 	CC

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

@@ -507,6 +507,12 @@ General-purpose environment variables:
 		The directory where the go command will write
 		temporary source files, packages, and binaries.
 
+Each entry in the GOFLAGS list must be a standalone flag.
+Because the entries are space-separated, flag values must
+not contain spaces. In some cases, you can provide multiple flag
+values instead: for example, to set '-ldflags=-s -w'
+you can use 'GOFLAGS=-ldflags=-s -ldflags=-w'.
+
 Environment variables for use with cgo:
 
 	CC