Correct the documentation for the `wp_kses_allowed_html` filter

[johnbillion]

The documentation for the wp_kses_allowed_html filter mistakenly documents the first parameter as the context instead of as the array of allowed HTML tags. This is due to the fact that the $context parameter can be used as either, and the filter falls within the condition where $context is actually an array of HTML tags.

Improving the variable names passed to the canonical instance of the filter helps clarify this. The six further instances of the filter immediately below it all reference the canonical documentation.

Trac ticket: https://core.trac.wordpress.org/ticket/53597

[johnbillion]

In 51675:

Security: Correct the inline docs for the wp_kses_allowed_html filter.

Props peterwilsoncc

Fixes #53597

https://core.trac.wordpress.org/changeset/51675