go/doc: exclude lines ending in ':' from possible headings

This is a more conservative approach to heading detection and
removes 11 headings from the current repository (several in
fmt). The current headscan output is:

/home/gri/go3/src/cmd/goinstall (package documentation)
        Remote Repositories
        The GOPATH Environment Variable
/home/gri/go3/src/pkg/exp/gotype (package documentation)
        Examples
/home/gri/go3/src/pkg/html/template (package template)
        Introduction
        Contexts
        Errors
        A fuller picture
        Contexts
        Typed Strings
        Security Model
/home/gri/go3/src/pkg/text/template (package template)
        Actions
        Arguments
        Pipelines
        Variables
        Examples
        Functions
        Associated templates
        Nested template definitions
18 headings found

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/5437105

src/pkg/go/doc/comment.go

@@ -241,8 +241,8 @@ func unindent(block []string) {
 	}
 }
 
-// heading returns the (possibly trimmed) line if it passes as a valid section
-// heading; otherwise it returns the empty string. 
+// heading returns the trimmed line if it passes as a section heading;
+// otherwise it returns the empty string. 
 func heading(line string) string {
 	line = strings.TrimSpace(line)
 	if len(line) == 0 {
@@ -255,17 +255,12 @@ func heading(line string) string {
 		return ""
 	}
 
-	// it must end in a letter, digit or ':'
+	// it must end in a letter or digit:
 	r, _ = utf8.DecodeLastRuneInString(line)
-	if !unicode.IsLetter(r) && !unicode.IsDigit(r) && r != ':' {
+	if !unicode.IsLetter(r) && !unicode.IsDigit(r) {
 		return ""
 	}
 
-	// strip trailing ':', if any
-	if r == ':' {
-		line = line[0 : len(line)-1]
-	}
-
 	// exclude lines with illegal characters
 	if strings.IndexAny(line, ",.;:!?+*/=()[]{}_^°&§~%#@<\">\\") >= 0 {
 		return ""

src/pkg/go/doc/comment_test.go

@@ -18,7 +18,8 @@ var headingTests = []struct {
 	{"Foo 42", true},
 	{"", false},
 	{"section", false},
-	{"A typical usage:", true},
+	{"A typical usage:", false},
+	{"This code:", false},
 	{"δ is Greek", false},
 	{"Foo §", false},
 	{"Fermat's Last Sentence", true},
@@ -26,7 +27,7 @@ var headingTests = []struct {
 	{"'sX", false},
 	{"Ted 'Too' Bar", false},
 	{"Use n+m", false},
-	{"Scanning:", true},
+	{"Scanning:", false},
 	{"N:M", false},
 }