a few javascript.html documentation edits

- A comma before "or" is only used when joining two independent clauses.
- "This looks like this" is redundant and a colon can introduce the
example just as well.
- Phrases beginning with "there may be," and other "there" variations,
can usually
be compressed.
- For consistency, changed wordy passive voice "has been called" for "is
called" (still passive voice, but with fewer characters).
- Changed "it may be desirable" to "you may want to"; sounds less like
one is wearing one's monocle.