Document addonHandler.initTranslation (#16132)

Fixes #15318

Summary of the issue:
Some users complained that calling addonHandler.initTranslation from the scratchpad raises an error.

Description of user facing changes
Document the function and update the User Guide to clarify that it is expected result.

projectDocs/dev/developerGuide/developerGuide.t2t

@@ -877,8 +877,9 @@ All other fields will be ignored.
 
 +++ Locale-specific Messages +++
 Each language directory can also contain gettext information, which is the system used to translate the rest of NVDA's user interface and reported messages.
-As with the rest of NVDA, an nvda.mo compiled gettext database file should be placed in the LC_MESSAGES directory within this directory.
-To allow plugins in your add-on to access gettext message information via calls to _(), you must initialize translations at the top of each Python module by calling addonHandler.initTranslation().
+As with the rest of NVDA, an ``nvda.mo`` compiled gettext database file should be placed in the ``LC_MESSAGES`` directory within this directory.
+To allow plugins in your add-on to access gettext message information via calls to ``_()``, ``ngettext()``, ``npgettext()`` and ``pgettext()`` you must initialize translations at the top of each Python module by calling ``addonHandler.initTranslation()``.
+This function cannot be called in modules that do not belong to an add-on, e.g. in a scratchpad subdirectory.
 For more information about gettext and NVDA translation in general, please read the [Translating NVDA wiki page https://github.com/nvaccess/nvda/wiki/Translating]
 
 ++ Add-on Documentation ++[AddonDoc]

source/addonHandler/__init__.py

@@ -833,6 +833,13 @@ def getCodeAddon(obj=None, frameDist=1):
 
 
 def initTranslation():
+	"""Initializes the translation of an add-on so that the Gettext functions (_, ngettext, npgettext and
+	pgettext) point to the add-on's translation rather than to NVDA's one.
+	This function should be called at the top of any module containing translatable strings and belonging to
+	an add-on. It cannot be called in a module that does not belong to an add-on (e.g. in a subdirectory of the
+	scratchpad).
+	"""
+
 	addon = getCodeAddon(frameDist=2)
 	translations = addon.getTranslationsInstance()
 	_TRANSLATION_FUNCTIONS = {