[NFC][analyzer][docs] Improve Annotations.rst (#122749)

[NFC][analyzer][docs] Improve Annotations.rst (#122749)

This commit fixes three issues within the documentation file
`Annotations.rst` which was recently created by my earlier commit
https://github.com/llvm/llvm-project/pull/122246 .

(1) The section title "Annotations to Enhance Generic Checks" is changed
to "General Purpose Annotations" because it was a bit too verbose and it
used the obsolete name "checks" for what we now call "checkers" in the
static analyzer.

(2) Several code blocks were missing from the generated html because I
accidentally used `.. code-block: c` instead of `.. code-block:: c` and
so Sphinx parsed them as comment blocks. (Without printing any error or
warning...)

(3) The `ownership_*` attributes (which are used by `MallocChecker`)
were missing from this document, so I wrote a section that briefly
describes them and links to their full documentation.