Partial rewrite:

Partial rewrite:
* Integrate the leftovers of the former NOTES section into the main text,
resulting in a more logical order of information.
* Make many descriptions more precise and tweak many wordings.
For example, the description of OBJ_cmp(3) was totally misleading.

Add a CAVEATS section explaining the scary ownership contracts
of the functions returning ASN1_OBJECT pointers.

Move the discussion of NID_undef to the BUGS section because the
statement "objects which are not in the table have the NID value
NID_undef" was misleading in more than one way.

Considering that an API as fundamental as this one contains such a
gigantic amount of quirks and traps and gaps makes me shudder.