Document some manifest build options

The --extra-manifest-xml and --extra-manifest-application-arguments
options are currently only handled by the sdl2 bootstrap while the
--manifest-placeholders option is handled in the common build.gradle
template.

Author: dbnicholson

doc/source/buildoptions.rst

@@ -91,6 +91,12 @@ options (this list may not be exhaustive):
 - ``--add-source``: Add a source directory to the app's Java code.
 - ``--no-compile-pyo``: Do not optimise .py files to .pyo.
 - ``--enable-androidx``: Enable AndroidX support library.
+- ``--extra-manifest-xml``: Extra xml to write directly inside the
+  ``<manifest>`` element of AndroidManifest.xml.
+- ``--extra-manifest-application-arguments``: Extra arguments to be
+  added to the ``<manifest><application>`` tag of AndroidManifest.xml.
+- ``--manifest-placeholders``: Inject build variables into the manifest
+  via the ``manifestPlaceholders`` property.
 - ``--enable-google-services``: Enable the Google Services Gradle plugin.
   This option requires a ``google-services.json`` file in root of the
   project directory.
@@ -160,6 +166,8 @@ ready.
 - ``add-source``: Add a source directory to the app's Java code.
 - ``--port``: The port on localhost that the WebView will
   access. Defaults to 5000.
+- ``--manifest-placeholders``: Inject build variables into the manifest
+  via the ``manifestPlaceholders`` property.
 - ``--enable-google-services``: Enable the Google Services Gradle plugin.
   This option requires a ``google-services.json`` file in root of the
   project directory.