doc/radosgw: Improve language, capitalization and use config database

doc/radosgw: Improve language, capitalization and use config database

Use "RADOS Gateway" instead of "Rados Gateway", "rados gateway" etc.
I am aware of the term "Ceph Object Gateway" but this change intends to
be an uncontroversial low hanging fruit fix of obviously incorrectly
capitalized terms.

Use "RGW daemon" instead of "Gateway", "Rados Gateway" etc.
Use "RGW instance" instead of "rados gateway" for consistency with
exactly similar other instance.
If referring obviously clearly to an instance of the daemon with an
obviously not preferred term, change it to "RGW daemon"; for example
when talking about restarting the RGW.
Do not touch other instances that are not 100% clear.

The files touched mostly do not use "Ceph Object Gateway" so changing
the term to it would create inconsistency, or several more changes
would need to be done to update all instances to use this terminology.

Use configuration database instead of ceph.conf in d3n_datacache.rst.

Improve language in d3n_datacache.rst.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>