Migrate the BTextMailComponent docs to the Haiku Book.

Migrate the BMailComponent docs to the Haiku Book.

I've updated the docs to match the current BMailComponent, documented
new functions, and cleaned up the MailComponent.h file to at least
somewhat m

Migrate the BMailComponent docs to the Haiku Book.

I've updated the docs to match the current BMailComponent, documented
new functions, and cleaned up the MailComponent.h file to at least
somewhat match our coding style.

First in a series (there are 3 more old API docs on the Mail Kit in that
"Public API" folder.)

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