ZIM name vs Book name confusion in documentation

[veloman-yunkan]

kiwix-serve documentation has incorrectly merged two ways of referring to a library entry. In the current implementation there are two kind of names associated with a ZIM file:

1. An identifier derived from the file path.
2. An identifier recorded in the ZIM archive metadata, library XML file or the OPDS feed (only one of these is associated with a book entry at a time).

As an example of a mistake in the documentation, it follows that the books.name and books.filter.name filtering parameters are equivalent, whereas the former filters by identifier#1 from above and the latter by identifier#2.

I will fix the documentation but I think that we should also try to eliminate the source of such confusion.