Docs: add a mention about the use of `loading` attribute in `wp_get_attachment_image` function

[audrasjb]

wp_get_attachment_image Doc Block should mention that the loading attribute can be used in the $attr parameter.

This parameter was introduced in ticket #50425.

[audrasjb]

Media: Add proper documentation about the use of the loading attribute in the attr parameter of wp_get_attachment_image

[SergeyBiryukov]

In 48852:

Docs: Add documentation and a @since note about the loading attribute added to wp_get_attachment_image().

Follow-up to [48272].

Props audrasjb.
Fixes #51122.

[SergeyBiryukov]

In 48853:

Docs: Add a @since note about the $srcset and $sizes attributes added to wp_get_attachment_image().

Follow-up to [34855].

See #51122.

[SergeyBiryukov]

In 48854:

Docs: Add documentation and a @since note about the loading attribute added to wp_get_attachment_image().

Follow-up to [48272].

Props audrasjb.
Merges [48852] and [48853] to the 5.5 branch.
Fixes #51122.