Updated dpkg-gensymbols's man page with lots of information

Updated dpkg-gensymbols's man page with lots of information

Explain the new include mechanism and in which cases it can be useful.
Also explain the need to keep the symbols file up-to-date. Include
recommendations to verify that upstream changes the library in sane ways.
This probably needs to be expanded, but probably in an external non-dpkg
related document that we could point to.