doc/how-to/fixtures: rework confusing usage of backticks

Using backticks on the words "override", "global", "root", "locally"
makes it look like they are code keywords.

I went ahead and also reworded it a bit, as overriding is not only for
global fixtures, and not necessarily "locally".

Author: bluetech

doc/en/how-to/fixtures.rst

@@ -1760,8 +1760,8 @@ into a configuration file:
 Overriding fixtures on various levels
 -------------------------------------
 
-In relatively large test suite, you most likely need to ``override`` a ``global`` or ``root`` fixture with a ``locally``
-defined one, keeping the test code readable and maintainable.
+In relatively large test suite, you may want to *override* a fixture, to augment
+or change its behavior inside of certain test modules or folders.
 
 Override a fixture on a folder (conftest) level
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^