fix RST parsing after option lists

[a-mr]

Fixes a minor bug with parsing failure after an option list finished (ref #17340 ).

An example from nimc.rst/basicopt.txt:

Command:
  //compile, c              compile project with default code generator (C)
  //r                       compile to $nimcache/projname, run with [arguments]
                            using backend specified by `--backend` (default: c)
  //doc                     generate the documentation for inputfile for
                            backend specified by `--backend` (default: c)

Arguments:
  arguments are passed to the program being run (if --run option is selected)

Before (nim stable, nim devel):

After:

Another similar bug:

* a
  -m   desc
  -n   very long
       desc
-d  option

Before:

After:

(Found while working on #17415 .)

• Also documented a rule for working with indentation and p.idx advancing in rst.nim
• Also documented Nim-specific double-slash // option

cc @narimiran