HTML API: Restore mistakenly-removed content in documentation.

[dmsnell]

In a3ed152 the Unicode replacement character was mistakenly removed, being believed to be a text rendering mistake, because the character is the replacement character used in situations where those text mistakes occur.

However, the purpose of including the character was to communicate what it looks like and why the Tag Processor won't insert it into the document (which would be wrong from the Unicode point of view for such a streaming processor that doesn't produce the final rendered view of the text).

In this patch we're bringing back the character and adding a small clue to try and remove the confusion that previously led to its unwanted removal.

Trac: #58256-ticket

In a3ed152c54c117d39d32a72a3d22f082dd5ffa7f the Unicode replacement character was mistakenly removed, being believed to be a text rendering mistake, because the character _is_ the replacement character used in situations where those text mistakes occur.

However, the purpose of including the character was to communicate what it looks like and why the Tag Processor won't insert it into the document (which would be wrong from the Unicode point of view for such a streaming processor that doesn't produce the final rendered view of the text).

In this patch we're bringing back the character and adding a small clue to try and remove the confusion that previously led to its unwanted removal.

[dmsnell]

cc: @johnbillion @zieladam

[audrasjb]

Thanks for the ticket and patch. Looks good to me.

Introduced in [55718]/#57840 (milestone 6.3, so version is trunk).

[audrasjb]

In 55723:

HTML API: Restore mistakenly-removed content in documentation.

In [55718] the Unicode replacement character was mistakenly removed. The purpose of including the character was to communicate what it looks like and why the Tag Processor won't insert it into the document.

This changeset brings the character back and adds a small clue to fix the confusion that may lead to its removal.

Follow-up to [55718].

Props dmsnell.
Fixes #58256
See #57840.

Committed in https://core.trac.wordpress.org/changeset/55723