Changes boilerplate sentence of "If using a field as the argument, this
parameter only supports..." to "...this parameter supports only...".
The latter is a bit more clear and readable.
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
