tree: Fix various typos and outdated terminology

This was mostly machine generated with the following command:
```
codespell \
    --builtin clear,rare,usage \
    --skip './po/*' --skip './.git/*' --skip './NEWS*' \
    --write-changes .
```
using the latest git version of `codespell` as per [these
instructions](https://github.com/codespell-project/codespell#user-content-updating).

Then I manually checked each change using `git add -p`, made a few
manual fixups and dropped a load of incorrect changes.

There are still some outdated or loaded terms used in GLib, mostly to do
with git branch terminology. They will need to be changed later as part
of a wider migration of git terminology.

If I’ve missed anything, please file an issue!

Signed-off-by: Philip Withnall <withnall@endlessm.com>

docs/reference/gio/overview.xml

@@ -281,7 +281,7 @@ be careful. GIO has extension points whose implementations get loaded
 from modules (executable code in shared objects), which could allow
 an attacker to sneak his own code into your application by tricking it
 into loading the code as a module. However, GIO will never load modules
-from your home directory except when explictly asked to do so via an
+from your home directory except when explicitly asked to do so via an
 environment variable.
 </para>