cmd/go: reapply doc change from CL 60590044.

https://golang.org/cl/60590044 edited
doc.go without editing the file it is generated from.
The edit was lost at the next mkdoc.sh.
Make the change in help.go and rerun mkdoc.sh.

Pointed out in the review of CL 68580043.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/88760043

src/cmd/go/doc.go

@@ -511,8 +511,8 @@ http://swig.org/.  When running go build, any file with a .swig
 extension will be passed to SWIG.  Any file with a .swigcxx extension
 will be passed to SWIG with the -c++ option.
 
-When either cgo or SWIG is used, go build will pass any .c, .s, or .S
-files to the C compiler, and any .cc, .cpp, .cxx files to the C++
+When either cgo or SWIG is used, go build will pass any .c, .m, .s,
+or .S files to the C compiler, and any .cc, .cpp, .cxx files to the C++
 compiler.  The CC or CXX environment variables may be set to determine
 the C or C++ compiler, respectively, to use.
 

src/cmd/go/help.go

@@ -19,8 +19,8 @@ http://swig.org/.  When running go build, any file with a .swig
 extension will be passed to SWIG.  Any file with a .swigcxx extension
 will be passed to SWIG with the -c++ option.
 
-When either cgo or SWIG is used, go build will pass any .c, .s, or .S
-files to the C compiler, and any .cc, .cpp, .cxx files to the C++
+When either cgo or SWIG is used, go build will pass any .c, .m, .s,
+or .S files to the C compiler, and any .cc, .cpp, .cxx files to the C++
 compiler.  The CC or CXX environment variables may be set to determine
 the C or C++ compiler, respectively, to use.
 	`,