Optional parameters in sanitize_post_field() aren't declared as optional.

[morganestes]

The docblock and internals of sanitize_post_field() say the $context parameter has a default, but the function signature has it as a required parameter. Since the other related functions have a default value, adding one here makes it clearer what is happening here.

[DrewAPicture]

@morganestes: We'd also need a changelog entry in the DocBlock for making the parameter optional.

[morganestes]

Updated with @since documeted.

[wonderboymusic]

In 34081:

sanitize_post(): the default value for $context is 'display'. The documentation says the default for sanitize_post_field() is 'display', but there is no default for the arg. Make the argument: $context = 'default'.

Props morganestes.
Fixes #33117.