go/doc: allow interior dot in heading, as in "go.mod"

Only the expected headings are affected.
Diffing the output of "go run headscan.go" before and after:

$ diff head1 head2
26a27,28
> 	Edit go.mod from tools or scripts
> 	Make go.mod semantically consistent
168c170
< 141 headings found
---
> 143 headings found
$

Fixes #26938.

Change-Id: I204fd982a60773aa26880cd19eed890c373b8ab6
Reviewed-on: https://go-review.googlesource.com/129677
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/doc/comment.go

@@ -232,7 +232,7 @@ func heading(line string) string {
 	}
 
 	// exclude lines with illegal characters. we allow "(),"
-	if strings.ContainsAny(line, ".;:!?+*/=[]{}_^°&§~%#@<\">\\") {
+	if strings.ContainsAny(line, ";:!?+*/=[]{}_^°&§~%#@<\">\\") {
 		return ""
 	}
 
@@ -248,6 +248,18 @@ func heading(line string) string {
 		b = b[i+2:]
 	}
 
+	// allow "." when followed by non-space
+	for b := line;; {
+		i := strings.IndexRune(b, '.')
+		if i < 0 {
+			break
+		}
+		if i+1 >= len(b) || b[i+1] == ' ' {
+			return "" // not followed by non-space
+		}
+		b = b[i+1:]
+	}
+
 	return line
 }