Change default value of $context to empty string for request_filesystem_credentials() and get_filesystem_method()

[ocean90]

$context is a path to a directory and shouldn't be false by default.

Automatic_Upgrader_Skin::request_filesystem_credentials() and WP_Upgrader_Skin::request_filesystem_credentials() need to be updated too.

[ocean90]

@DrewAPicture How does 37412.2.patch​ look to you?

[DrewAPicture]

@ocean90 Since we're changing the default for $context, 37412.diff​ adds changelog entries to the function DocBlocks and as well as DocBlocks for hooks that would additionally be affected by the change.

It might seem like a lot of ground to cover, but anybody relying on $context defaulting to false in the resulting hooks should get some kind of explanation.

[ocean90]

In 38138:

Filesystem API: Change the default value for the $context parameter of get_filesystem_method() and request_filesystem_credentials() to an empty string.

$context is a full path to the directory that is tested for being writable. A path shouldn't be a boolean value.
This also updates WP_Upgrader_Skin::request_filesystem_credentials() and Automatic_Upgrader_Skin::request_filesystem_credentials() and adds missing docs.

Props DrewAPicture, ocean90.
Fixes #37412.