update usage/docs for cmds with options `-L, --color[=WHEN]` to indicate that `-L` can't take the `WHEN` argument and `=` is required about flux-core HOT 2 CLOSED

i was admittedly surprised this didn't work. Some minor googling does suggest optional short options is a GNU only extension, thus something we maybe don't want to support in general.

I believe we copied this option from ls and ls only supports the long option. Perhaps we should just only support the long option. The long option is only documented/supported for a few commands already, so removing L short option would support command consistency.

man1/flux-dmesg.rst:.. option:: -L, --color[=WHEN]
man1/flux-jobs.rst:.. option:: --color[=WHEN]
man1/flux-kvs.rst:.. option:: -L, --color[=WHEN]
man1/flux-kvs.rst:.. option:: -L, --color[=WHEN]
man1/flux-overlay.rst:.. option:: -L, --color=WHEN
man1/flux-top.rst:.. option:: --color[=WHEN]

perhaps need a NO_SHORT_OPTION macro of sorts of liboptparse. (Edit ..oh if we don't list a short option key, it appears only long option works).

Edit:

some auditing notes for myself
dmesg - liboptparse
jobs - argparse
kvs - liboptparse
overlay - liboptparse
flux-top - liboptparse

from flux-core.

I believe we copied this option from ls and ls only supports the long option.

We also copied dmesg(8) which supports the short option:

Usage:
 dmesg [options]

Display or control the kernel ring buffer.
[snip]
 -H, --human                 human readable output
 -k, --kernel                display kernel messages
 -L, --color[=<when>]        colorize messages (auto, always or never)
                               colors are enabled by default
 -l, --level <list>          restrict output to defined levels
...

Let's keep the short option at least in utilities that are likely to be piped to less -R, e.g. flux dmesg, flux *eventlog, etc.

from flux-core.