Documentation for the pre_get_avatar hook

[henry.wright]

Passing a non-null value will effectively short-circuit get_avatar(), passing the value through the {@see 'pre_get_avatar'} filter and returning early.

I believe the value is actually passed through the get_avatar hook, not pre_get_avatar.

[henry.wright]

Alternatively of course, the documentation could be correct but the name of the actual filter hook could be wrong. Either way, there's an issue :)

[SergeyBiryukov]

In 36800:

Docs: Correct filter reference in pre_get_avatar filter description.

Props henry.wright.
Fixes #36031.