Documentation: irq-domain.rst: Simple improvements

The improvements include:

  * Capitals in headlines.
  * Add commas: for easier reading, it is always desired to add commas
    at some places in text. Like before adverbs or after fronted
    sentences.
  * 3rd person -> add 's' to verbs.
  * End some sentences with period and start a new one. Avoid thus heavy
    sentences.

[ tglx: Fix up subject prefix ]

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250319092951.37667-56-jirislaby@kernel.org