Tweak configuration docs

This PR contains suggested changes to the configuration docs. Some things included are:

Removal of mention of the obsolete thread module
Updating the list of os functions available that remain when JANET_REDUCED_OS is set
Updating the list of peg functions that get disabled when JANET_NO_PEG is set
Minor spelling / grammatical / wording tweaks
Some wrapping changes for width-challenged situations

janet-lang / janet-lang.org