Doc: Info: Revise dir entries and make process

This request contains two minor edits, a redistribution of the dir file category and entry data, and a simplification of the install/uninstall process. The edits are self explanatory.

The modification of dir data improves the odds that LilyPond info files will be correctly packaged in Linux distributions. Further explanation is given in the commit message.

Given changes to dir data, the install/uninstall process can be simplified to unify handling of /usr and non-/usr, e.g., /opt/..., installation sites. This allows to remove the warning to packagers to post process info files using a command that fails if a simple a "make ; make install" is done.