HaikuBook: fix part of the warnings marked by Doxygen

I spotted some warnings when I upgraded from Doxygen 1.18.13 to 1.18.17.
The new warnings are useful, they point out imbalances in grouping, as
well as unclosed comment blocks. Coincidentally, this fixes #13338

BEntry::Unset doc: missing \fn, better description.

Documentation updates for Storage Kit classes

* Added \since to each method and parameter.
* Whitespace cleanup.
* Some other minor cleanups and updates.

Big docs cleanup.

* Fixed headers including:
- All rights reserved not All Rights Reserved.
- name, email@domain.com not name <email@domain.com>
- tabs and spaces
- Authors: not Documented by:
* Renamed string.dox to String.dox
* Renamed midixxx.dox files to MidiXxx.dox
* Moved images into images subdirectories and updated Doxfile.
* Re-format all files with tabs instead of spaces.
* Fix many spelling mistakes.
* Added all files, classes, structs, and enums to libbe group.

Add \brief tag so description appears in files table

A few formatting tweaks in the BEntry and BDirectory class documentation.

Moved documentation for the BEntry class into a dox file to include it into the Haiku book.

BEntry::Unset doc: missing \fn, better description.

Documentation updates for Storage Kit classes

* Added \since to each method and parameter.
* Whitespace cleanup.
* Some other minor cleanups and updates.

Big docs cleanup.

* Fixed headers including:
- All rights reserved not All Rights Reserved.
- name, email@domain.com not name <email@domain.com>
- tabs and spaces
- Authors: not Documented by:
* Renamed string.dox to String.dox
* Renamed midixxx.dox files to MidiXxx.dox
* Moved images into images subdirectories and updated Doxfile.
* Re-format all files with tabs instead of spaces.
* Fix many spelling mistakes.
* Added all files, classes, structs, and enums to libbe group.

Add \brief tag so description appears in files table

A few formatting tweaks in the BEntry and BDirectory class documentation.

Moved documentation for the BEntry class into a dox file to include it into the Haiku book.