Simplify wording and add some formatting in several locations.

Expand sentinel array tests to highlight (non-)handling of internal
sentinels.

Fix format of symbol names in function pointers example.

Clarify wording a bit on the builin atomic* documentation.

Remove the (second) builtin compileLog example that demonstrated a lack of
compileLog entries.

* langref: address comments from rohlem

Use "0-terminated" instead of "null-terminated".

Undo some changes that were not as clear an improvement as I though.

* langref: remove stray ""

Thanks to rohlem for spotting this typo.