fix typos in documentation

[bcr]

We can't make changes to past release notes once they are published. They become part of the release hash and would change in a number of places that reference them (even if they contain typos).

[ziaee]

In D58001#1330236, @bcr wrote:

We can't make changes to past release notes once they are published. They become part of the release hash and would change in a number of places that reference them (even if they contain typos).

I was under the impression that this was only for the announcement and we can fix the release notes themselves.

[vladlen]

In D58001#1330236, @bcr wrote:

We can't make changes to past release notes once they are published. They become part of the release hash and would change in a number of places that reference them (even if they contain typos).

There was a recent discussion in releng (I don't recall
if it was on the mailing list or a thread CC'd to [e-mail])
about whether we can modify old relnotes.adoc and hardware.adoc
when there's an obvious error. The conclusion was that since neither
of these documents is cryptographically signed now (only announcement),
they can be changed. relnotes contains important information for users
and should not be misleading.

Also, note that the old release notes were mechanically converted
from HTML -> DocBook -> AsciiDoc.

That said, I'm not insisting on this — I'm open to other opinions.
Let's make sure this procedure is done correctly.

P.S. I ran spellcheck on all documents in the website tree,
but I excluded relnotes for 8.0R and earlier (considering
them historical artifacts) as well as signed files like
announcements and security advisories.