Document basic TeXbook terms required by LaTeX3

[josephwright]

Things like are needed to document a number of functions. Currently, we rely on The TeXbook for this, but in the longer term need to have a set of (formal) definitions.

[blefloch]

Subtleties that could be worth documenting:

• treatment of spaces (e.g., gotcha in ExplSyntax, \~~a is equivalent to \~a)
• what tokens can be (meaning, catcode)
• syntax of integer expressions, dimension expressions, skip expressions.

[josephwright]

Do we want to make this a separate file, say l3term-glossary? That can then be typeset separately, or integrated into interface3, or ....

[blefloch]

I did a tiny bit. Won't have time for more now but maybe we can be motivated to add to the file once in a while.