go/format: document use of Source better

For #22695.

Change-Id: Idcda3294070aeaeaf940aaf2014b573732fd60a4
Reviewed-on: https://go-review.googlesource.com/80696Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>

src/go/format/format.go

@@ -24,8 +24,8 @@ const parserMode = parser.ParseComments
 // The node type must be *ast.File, *printer.CommentedNode, []ast.Decl,
 // []ast.Stmt, or assignment-compatible to ast.Expr, ast.Decl, ast.Spec,
 // or ast.Stmt. Node does not modify node. Imports are not sorted for
-// nodes representing partial source files (i.e., if the node is not an
-// *ast.File or a *printer.CommentedNode not wrapping an *ast.File).
+// nodes representing partial source files (for instance, if the node is
+// not an *ast.File or a *printer.CommentedNode not wrapping an *ast.File).
 //
 // The function may return early (before the entire result is written)
 // and return a formatting error, for instance due to an incorrect AST.
@@ -79,6 +79,10 @@ func Node(dst io.Writer, fset *token.FileSet, node interface{}) error {
 // space as src), and the result is indented by the same amount as the first
 // line of src containing code. Imports are not sorted for partial source files.
 //
+// Caution: Tools relying on consistent formatting based on the installed
+// version of gofmt (for instance, such as for presubmit checks) should
+// execute that gofmt binary instead of calling Source.
+//
 func Source(src []byte) ([]byte, error) {
 	fset := token.NewFileSet()
 	file, sourceAdj, indentAdj, err := parse(fset, "", src, true)