[PATCH] Add backslash to list of POST characters for text markup

[Tom Alexander]

Backslash appears to be supported. To test I used the following test document:
```
foo ~bar~\& baz
```

This happens in a document in worg: 
https://git.sr.ht/~bzg/worg/tree/ae64e1a54185232d4ebdcab174d8d4319ffd564d/org-release-notes.org#L6666

The ampersand was chosen for the test document since that is not a supported 
POST character, to make sure backslash was not simply escaping the next 
character.

In the documentation I wrote out the word "backslash" in parenthesis to 
disambiguate between backslash and escaping the following comma.

Patch is attached.

--
Tom Alexander
pgp: https://fizz.buzz/pgp.asc

0001-Add-backslash-to-list-of-POST-characters-for-text-ma.patch
Description: Text Data