fix($filter): change documentation of $filter to reflect optional com…

[kylewuolle]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
documentation update

What is the current behavior? (You can also link to an open issue here)
In the $filter documentation the comparator parameter is not identified as being optional. See issue [15312] (#15312).

What is the new behavior (if this is a feature change)?
In the documentation for $filter the comparator parameter is now marked optional and the default value is identified as being false.

Does this PR introduce a breaking change?
No

Other information:

change the documentation for $filter so that the comparator parameter is marked optional. Also specified that the default value is false.

Closes #15312

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[kylewuolle]

I signed it

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