cmd/yacc: move cmd/yacc/expr to cmd/yacc/testdata/expr

This will keep the go command from trying to build it
when the cmd/ tree is no longer a special case.

Also update doc.go to refer to the correct location.
(It was incorrect even before this CL.)

LGTM=r
R=iant, r
CC=golang-codereviews
https://golang.org/cl/134560043

src/cmd/yacc/doc.go

@@ -20,8 +20,9 @@ written in C and documented at
 Adepts of the original yacc will have no trouble adapting to this
 form of the tool.
 
-The file expr.y in this directory is a yacc grammar for a very simple
-expression parser.  It needs the flag "-p expr" (see below).
+The directory $GOROOT/cmd/yacc/testdata/expr is a yacc program
+for a very simple expression parser. See expr.y and main.go in that
+directory for examples of how to write and build yacc programs.
 
 The generated parser is reentrant. Parse expects to be given an
 argument that conforms to the following interface: