Signed-off-by: NRémy Coutable <remy@rymai.me>

[ci skip]

Users must not change the default filename or they will have to create a
suitable .ssh/config which is out-of-scope here.

The keytype field is actually `ssh-dss` for DSA keys and they will not
be stored as `id_rsa.pub`.

Note that newer version of ssh actually also support
`ecdsa-sha2-nistp256` and others so it is also misleading to assume the
field always starts with `ssh-`.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/875

Provide help for people who aren't SSH-clued by indicating
how to point to key files that have been named something
other than the default. A lot of people seem to assume that
the filename they choose is irrelevant and then wonder why
their private key is not being found by simple 'ssh' commands.

available topics are quite short.

Also correct some missing words, punctuation.