Documentation: multiple .rst files: Fix grammar and more consistent formatting

sphinx.rst:
- Remove unnecessary newline
- Fix grammar s/on/in/
- Fix grammar s/check/checks/
- Capitalize heading "The C domain"

changes.rst:
- Remove colon after "pahole" to be consistent with other entries

howto.rst:
- Fix grammar s/you will/will you/
- Hyphenate "real-world problems"

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240205000117.3285-1-thorsten.blum@toblux.com