Explicitly document \001 and \002?

[Mingye Wang]

Hi,

Readline currently uses SOH (\001) and STX (\002) to label parts of
the text that does not use up screen space, but from the documentation
point of view it is hidden behind the C macros
`RL_PROMPT_START_IGNORE' and `RL_PROMPT_END_IGNORE'. This makes the
documentation inaccessible to people using readline from other
languages (including bash PS1 via command substitution), unless the
binding explicitly provides these constants.

I propose putting the values of the two definitions explicitly in the
readline documentation.for rl_expand_prompt. It is already partially
present in the user-interface documentation about the mode strings.
There shouldn't be any compatibility issues, as a change would require
the invoking program to be recompiled anyways.

Thanks,
Mingye Wang (Artoria2e5)