go/doc: compile regexps lazily

Compile go/doc's 4 regexps lazily, on demand.

Also, add a test for the one that had no test coverage.

This reduces init-time CPU as well as heap by ~20KB when they're not
used, which seems to be common enough. As an example, cmd/doc only
seems to use 1 of them. (as noted by temporary print statements)

Updates #26775

Change-Id: I85df89b836327a53fb8e1ace3f92480374270368
Reviewed-on: https://go-review.googlesource.com/127875


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>