Notes that you cannot use EQL in ES to search the values of `nested`
fields or their sub-fields. However, indices containing `nested` field
mappings are otherwise supported.
Adds documentation for the EQL `substring` function.
Supporting changes:
* Creates a new "EQL function reference" page
* Updates the title of the "EQL syntax reference" page for consistency
* Adds a brief "Functions" section to the EQL syntax docs
* Updates EQL limitations docs to state that only array functions are
unsupported
James Rodewig