Docs: Improve documentation about how relative file paths are resolved

Both when loading external service plugins and when loading custom
functions, relative paths are resolved from the folder of the
"mqttwarn.ini" configuration file.

Author: amotl

HANDBOOK.md

@@ -3063,13 +3063,22 @@ Below are a number of example scenarios where custom functions are being used.
 Consider the following configuration snippet in addition to the configuration
 of the `mqtt` service shown above:
 
+Add this to a custom Python file, like, e.g. `/etc/mqttwarn/myfunctions.py`.
 ```python
 def lookup_data(data, srv=None):
     if type(data) == dict and 'fruit' in data:
             return "Ananas"
     return None
 ```
 
+Register that file by saying:
+
+```ini
+[defaults]
+; path to file containing self-defined functions for formatmap and datamap
+functions = 'myfunctions.py'
+```
+
 Then, in the section defining the topic we listen on:
 
 ```ini
@@ -3079,17 +3088,24 @@ Then, in the section defining the topic we listen on:
 format =  lookup_data()
 ```
 
-We've replaced the `formatmap` entry for the topic by a function which you
-define within the _functions_ file you configure as `functions` in `mqttwarn.ini` configuration file. These functions
-are invoked with decoded JSON `data` passed to them as a _dict_. The string returned
-by the function returned string replaces the outgoing `message`:
+Here, we replaced the `formatmap` entry for the topic by a custom function.
+
+These functions are invoked with decoded JSON `data` passed to them as a
+_dict_. The string returned  by the function returned string replaces the
+outgoing `message`:
 
 ```
 in/a1 {"fruit":"pineapple", "price": 131, "tst" : "1391779336"}
 out/food Ananas
 out/fruit/pineapple Ananas
 ```
 
+You can define custom functions in a Python file which you configure as
+`functions` in the `[default]` section of the `mqttwarn.ini` configuration
+file, as outlined above. When relative file names are given, they will be
+resolved from the directory of the `mqttwarn.ini` file, which is, by default,
+the `/etc/mqttwarn` folder.
+
 If a function operating on a message (i.e. within `format =`) returns `None` or an empty string, the target notification is suppressed.
 
 The optional `srv` is an object with some helper functions. In particular, these allow us to use _mqttwarn_'s logging and MQTT publish functions, as in this example:
@@ -3412,7 +3428,12 @@ plugins, there are two options.
 ### Service plugin from package
 
 This configuration snippet outlines how to load a custom plugin from a Python
-module referenced in "dotted" notation.
+module referenced in "dotted" notation. Modules will be searched for in all
+directories listed in [`sys.path`]. Custom directories can be added by using the
+[`PYTHONPATH`] environment variable.
+
+[`sys.path`]: https://docs.python.org/3/library/sys.html#sys.path
+[`PYTHONPATH`]: https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH
 
 ```ini
 [defaults]
@@ -3433,7 +3454,9 @@ targets = {
 ### Service plugin from file
 
 This configuration snippet outlines how to load a custom plugin from a Python
-file referenced by file name.
+file referenced by file name. When relative file names are given, they will be
+resolved from the directory of the `mqttwarn.ini` file, which is, by default,
+the `/etc/mqttwarn` folder.
 
 ```ini
 [defaults]