Documentation/rexecd: document command-line options including -t

[JianyuWang0623]

Summary

The documentation page for the rexecd remote execution server
(Documentation/applications/netutils/rexecd/index.rst) previously
contained only a title and no content. This change fills it in with:

• A short description of the daemon: it serves remote execution
  requests from the rexec client, listens on the rexec port
  (REXECD_PORT, 512) for the IP families or on a corresponding RPMsg
  socket name for AF_RPMSG, authenticates the request, runs the
  command and pipes its output back to the client.
• A note that, by default, rexecd spawns a detached worker thread per
  accepted connection so multiple sessions can run concurrently.
• A Usage synopsis and an Options section documenting every
  command-line option: -4 (AF_INET, default), -6 (AF_INET6),
  -r (AF_RPMSG, with a :doc: cross-reference to the RPMsg core
  concepts page), and the -t threadless option, which serves each
  connection inline in the main task instead of spawning a
  per-connection worker thread.

The -t option is the recently added "threadless" mode: it avoids
allocating the worker stack (CONFIG_NETUTILS_REXECD_STACKSIZE) from
the heap, which helps on low-memory targets, at the cost of serving
connections strictly one at a time.

Impact

• Documentation only. No source code, build process, hardware, security
  or compatibility impact.
• Adds user-facing documentation for the rexecd daemon and its
  command-line options, including the previously undocumented -t
  option.

Testing

Pure documentation change, tested with make html.

• Host: Ubuntu 22.04, Python 3.10.12, Sphinx 8.1.3.
• Built the docs from nuttx/Documentation with make html.
• The rexecd page builds with no warnings or errors; the generated
  _build/html/applications/netutils/rexecd/index.html renders the
  description, usage, and all options correctly.
• The :doc: cross-reference to the RPMsg concepts page resolves to a
  valid link (../../../components/drivers/special/rpmsg/concepts.html,
  text "RPMsg Core Concepts"); no broken references.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• s698pm-dkit
• stm32-nucleo-f103rb