Consistently use "startup process"/"WAL sender" instead of "Startup process"/"WAL Sender" in comments and docs.

Hi,

In the code comments, it is being used as "Startup process" in the
middle of the sentences whereas in most of the places "startup
process" is used. Also, "WAL Sender" is being used instead of "WAL
sender". Let's be consistent across the docs and code comments.

Attaching a patch. Thoughts?

Regards,
Bharath Rupireddy.