docs: add details about number flags to the documentation.

It's clearly intended to be enabled by default:
- it's defined as default behaviour translate.h (NUM_DEFAULT)
- tr_languages.c sets many default values related to number processing
  that have no meaning unless langopts.numbers == 1.

It is also a more sensible default since most languages will want to
have number processing on. This makes adding new languages easier
because  adding an entry to tr_languages.c is unnecessary.

A negative side effect is that languages with partial number defines
might experience bugs when reading undefined numbers. This is a bug and
should be fixed.

This will have the side effect of enabling number processing for
languages that currently have it disabled. However, there shouldn't be
any.

Here's a way to check affected languages:
for voice in $(ESPEAK_DATA_PATH=`pwd` LD_LIBRARY_PATH=src:${LD_LIBRARY_PATH}
src/espeak-ng --voices | grep -v Languages | awk '{print $2}'); do
OUTPUT=$(ESPEAK_DATA_PATH=`pwd` LD_LIBRARY_PATH=src:${LD_LIBRARY_PATH}
src/espeak-ng -qx -v $voice "1 - 2 - 3 - 12 - 123") && echo "$voice:
$OUTPUT" ; done

These voices clearly benefit from enabling numbers (they already have
number rules in *_list):
ba, cmn (zh), hak, haw, ja, kok, nb, nci

Some languages are missing some definitions (like _12) in _list files.
It causes the program to skip some numbers.
Numbering needs to be turned off explicitly for:
jbo, mi, my, piqd, py, qu, quc, th, uz

Languages with no number rules at all:
chr, cv, he, nog, tk, ug