Add information about `exec_rule`, `exec_rule_expression`, and `exec_rule_text`

[alanking]

The information should at least exist as code comments.

Here is the code comment in the main repo describing their purposes: https://github.com/irods/irods/blob/05403d37e025677860f89272a3d497d6dfba7b34/server/re/include/irods/irods_re_plugin.hpp#L35-L38

For posterity:

// Notes about the exec_rule_* family of functions:
// - exec_rule: Executes dynamic PEPs and user-defined rules.
// - exec_rule_text: Used by irule exclusively.
// - exec_rule_expression: Used to execute delay() and remote() code.

Related to / Inspired by: irods/irods_docs#304

[alanking]

I see a couple of comments in here about a couple of these already, but they are buried. They should be more prominently displayed as a way of documenting these more visibly.