The Remarkable Markdown library does the following auto-replacements:

(c) (C) (r) (R) (tm) (TM) (p) (P) +-

Produces:

(c) (C) (r) (R) (tm) (TM) (p) (P) +-

Note that this can be annoying if you are trying to create a set of questions that have part (a), (b), and (c) since the (c) gets auto-replaced. This can be thwarted using a zero-width space:

(​c)

Produces:

(​c)

Thanks to Nigel who came up with this solution.

It collapses extra punctuation into standard forms:

test.....
test!!!!!
test?????
test,,,,,

Produces:

test.....
test!!!!!
test?????
test,,,,,

It produces the appropriate dash character:

a - b
a -- b
a --- b

Produces:

a - b
a -- b
a --- b

It will convert straight quotes to the appropriate open/close quotes:

"speech marks"
'single quotes'

Produces:

"speech marks"
'single quotes'