improve documentation for install_command (#2111)

Co-authored-by: Bernát Gábor <[email protected]>

Author: jugmac00

docs/changelog/2081.doc.rst

@@ -0,0 +1 @@
+The documentation of ``install_command`` now also mentions that you can provide arbitrary commands - by :user:`jugmac00`

docs/config.rst

@@ -306,6 +306,13 @@ Complete list of settings that you can put into ``testenv*`` sections:
     such as ``--pre`` (configured as ``pip_pre``) and potentially index-options from the
     deprecated :conf:`indexserver` option.
 
+    .. note::
+
+        You can also provide arbitrary commands to the ``install_command``. Please take care that these commands can be
+        executed on the supported operating systems. When executing shell scripts we recommend to not specify the script
+        directly but instead pass it to the appropriate shell as argument (e.g. prefer ``bash script.sh`` over
+        ``script.sh``).
+
 .. conf:: list_dependencies_command ^ ARGV ^ python -m pip freeze
 
     .. versionadded:: 2.4