unnecessarily gendered language in documentation

[Ada]

bug: Having gendered language in documentation reinforces the exclusion of women and non-cis folks from participating in the project.

I was reading through some documentation to diagnose some issues with multistores and came across these.

I've included some proposed alternatives as well.

doc/source/glossary.rst|929 col 42|
- used to verify that the user is who *he* says he is.
+ used to verify that the user is who they say they are.

doc/source/contributor/domain_model.rst|28 col 8|

- because *he* does not know what layers are used and works with a domain
+ because they do not know what layers are used and works with a domain

doc/source/admin/multistores.rst|169 col 33|

- ``CONF.enabled_backeds`` then *he* needs to remove reserved
+ ``CONF.enabled_backeds`` then they need to remove reserved

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/glance/+/812681

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/glance/+/812681
Committed: https://opendev.org/openstack/glance/commit/88d586f038b40a6881f76e125608f2c43b206630
Submitter: "Zuul (22348)"
Branch: master

commit 88d586f038b40a6881f76e125608f2c43b206630
Author: Cyril Roelandt <email address hidden>
Date: Wed Oct 6 15:33:46 2021 +0200

    Use singular they instead of "he/she" where possible

    Change-Id: I5a30a62f8d62e2c97c30e795317e95c207e196b9
    Closes-Bug: #1946166

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/glance 24.0.0.0rc1 release candidate.