doc: link to python_files from "Assertion Rewriting" (pytest-dev#6705)

Ref: pytest-dev#6377
Ref: #145

Author: blueyed

doc/en/writing_plugins.rst

@@ -181,6 +181,7 @@ done via a :pep:`302` import hook which gets installed early on when
 ``pytest`` starts up and will perform this rewriting when modules get
 imported.  However since we do not want to test different bytecode
 then you will run in production this hook only rewrites test modules
+themselves (as defined by the :confval:`python_files` configuration option)
 themselves as well as any modules which are part of plugins.  Any
 other imported module will not be rewritten and normal assertion
 behaviour will happen.