A run of lines that are indented with any number of spaces or tabs
format as a <pre> block. This commit fixes various doc comments
that format badly according to that (standard) rule.

For example, consider:

	// - List item.
	//   Second line.
	// - Another item.

Because the - lines are unindented, this is actually two paragraphs
separated by a one-line <pre> block. This CL rewrites it to:

	//  - List item.
	//    Second line.
	//  - Another item.

Today, that will format as a single <pre> block.
In a future release, we hope to format it as a bulleted list.

Various other minor fixes as well, all in preparation for reformatting.

For #51082.

Change-Id: I95cf06040d4186830e571cd50148be3bf8daf189
Reviewed-on: https://go-review.googlesource.com/c/go/+/384257


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>