Improve --rules=[section1,section2]

Now that --rules= and --single= allow multiple sections, some adjustments are required.

doc/OPTIONS mentions the syntax:

(bleeding-jumbo)doc $ git grep -n SECTIONn
OPTIONS:27:--single[=(SECTION[,SECTION2,....,SECTIONn]|:rule)] "single crack" mode
OPTIONS:75:--rules[=(SECTION[,SECTION2,....,SECTIONn]|:rule)]  enable word mangling rules

Nevertheless, the description could be enhanced.
E.g., instead of

Enables word mangling rules that are read from the specified section, which
defaults to [List.Rules:Wordlist] if not given.

use

Enables word mangling rules that are read from the specified
List.Rules section(s), default is [List.Rules:Wordlist] if not given.

Since , is now used to separate section names, bleeding-jumbo shouldn't allow section names using a comma.
For now, this seems to apply to List.Rules section names, but I think we could enforce this generally.
Also, it would be good if core john would at least warn when , appears in a section name.

Currently, we have this bleeding-jumbo behaviour:

(bleeding-jumbo)run $ head -n 3 john-local.conf 
[List.Rules:fd_test1,fd_test2]
$1
$2

$ ./john --stdout --wordlist --rules=fd_[tab]

completes to

$ ./john --shdout --wordlist --rules=fd_test1,fd_test2 

(bleeding-jumbo)run $ ./john --stdout --wordlist --rules=fd_test1,fd_test2 
No "fd_test1,fd_test2" mode rules found in ./john.conf

Since --single= and --rules= accept (lists of) rules instead of section names (when prefixed with :), we shouldn't allow section names starting with :.

This restriction should be added to core john as well, if possible.

Current bleeding-jumbo behaviour:

(bleeding-jumbo)run $ head -n 3 john-local.conf 
[List.Rules::fd_test1]
$1
$2
(bleeding-jumbo)run $ ./john --stdout --wordlist --rules=::fd_test1 
Invalid rule in Command Line Rule at line 0: Invalid position code

Start with doc/OPTIONS and the existing --rules= and --single= command-line parsing paths. Reproduce the comma-containing and colon-prefixed section-name examples, then trace how core and bleeding-jumbo resolve sections; done means the documentation explains lists clearly and the requested invalid-name handling is covered.