Dead link on "debug_fwrite" page

[Rahmohn]

The ​debug_fwrite function page has a link to error_log function that doesn't exists. Probably correct link is to the page ​error_log on PHP website.

[coffee2code]

In 8487:

Developer: Handle exception to deprecated item that refers to an externally-linked resource as its preferred alternative.

Specifically, debug_fwrite() had been deprecated in favor of PHP's error_log(). In every other case, the implicit assumption is that the preferred alternative is another WP resource.

Props rahmohn, coffee2code.
Fixes #4310.

[Rahmohn]

@coffee2code

There is one more dead link in section ​Descrition. Unfortunately I saw that after you had fixed the first one.

[coffee2code]

In 8525:

Developer: Move auto-link exception handling into link_internal_element() so as to apply in a broader range of contexts.

Previous implementation only affected auto-linking in deprecation notice, but the need for the exception could also apply in text, param descriptions, see alsos, etc.

Props rahmohn, coffee2code.
See #4310.