Consistently handle trailing slashes in subpage links.

Consistently handle trailing slashes in subpage links.

The link text for [[/Foo/]] is `Foo` and the link text for
[[../Foo/]] is `Foo`.  So far so good.

But the link text for [[/Foo//]] is `Foo` while the link text
for [[../Foo//]] is `Foo/`.  We are stripping all trailing slashes
in the first case, but not the second.

Fix the code so that we strip all trailing slashes in both cases.
Update some of the comments in the code while we are at it.

Change-Id: Id61eacafea9820c404699a7902c8eb8102779516