Update modifiers in bibtex key pattern

[tobiasdiez]

The list of possible modifiers should be extended to include at least a reference to the list of formatters. Moreover, the regex formatter needs special documentation (I think the syntax is [author:regexmyregex] where myregex is the regular expression). Triggered by https://github.com/JabRef/jabref/issues/3582.

List of all available modifiers: https://github.com/JabRef/jabref/blob/7895ae07443612c63fee950ddee88650fecb4d91/src/main/java/org/jabref/logic/formatter/Formatters.java#L35-L64

[koppor]

In case someone works at that topic, one could investigate the functions of our zotero friends and maybe adapt the documentation or the implementation: https://retorque.re/zotero-better-bibtex/citation-keys/