xmerge: javadoc maintenance and minor code formatting

Javadoc:

* Replaces <code>..</code> tags with {@code ..}.
* Replaces HTML entities with {@literal ..}
* Removes <p>..</p> tags in the first sentence.
* Reduces code snippet spacing.
* Adds tables.
* Uses only one space after a comment delimiter.
* Adjusts the margins.

Minor code formating:

* Removes some extra blank lines (very few).
* Fixes some indentations (very few).
* Adjusts the margins in some method headers (very few).

Change-Id: I01bd9df58e4cb41bc3a38a2b2d3d7d2612081c32
Reviewed-on: https://gerrit.libreoffice.org/11202
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>