go/build: make interaction between file names and +build lines clearer

Change-Id: I2cae17d3f0d208c7ed1089bc5cb8f81022fcd36e
Reviewed-on: https://go-review.googlesource.com/6470Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/go/build/doc.go

@@ -111,7 +111,7 @@
 // (example: source_windows_amd64.go) where GOOS and GOARCH represent
 // any known operating system and architecture values respectively, then
 // the file is considered to have an implicit build constraint requiring
-// those terms.
+// those terms (in addition to any explicit constraints in the file).
 //
 // To keep a file from being considered for the build:
 //