Add documentation for IPHONEOS_DEPLOYMENT_TARGET usage.

Author: freakboy3742

docs/reference/environment-variables.rst

@@ -61,6 +61,21 @@ Environment variables used by meson-python
 .. _cross build definition file: https://mesonbuild.com/Cross-compilation.html
 .. _cibuildwheel: https://cibuildwheel.readthedocs.io/en/stable/
 
+.. envvar:: IPHONEOS_DEPLOYMENT_TARGET
+
+   This environment variable is used to specify the target iOS platform version
+   to the Xcode development tools.  If this environment variable is set,
+   ``meson-python`` will use the specified iOS version for the Python wheel
+   platform tag, instead than the iOS platform default of 13.0.
+
+   This variable must be set to a major/minor version (e.g., ``13.0`` or
+   ``15.4``).
+
+   Another way of specifying the target iOS platform is to use the
+   ``-mios-version-min`` compile and link flags.  However, it is not possible
+   for ``meson-python`` to detect this, and it will not set the Python wheel
+   platform tag accordingly.
+
 .. envvar:: FORCE_COLOR
 
    Setting this environment variable to any value forces the use of ANSI
@@ -69,11 +84,10 @@ Environment variables used by meson-python
 
 .. envvar:: MACOSX_DEPLOYMENT_TARGET
 
-   This environment variables is used of specifying the target macOS platform
-   major version to the Xcode development tools.  If this environment variable
-   is set, ``meson-python`` will use the specified macOS version for the
-   Python wheel platform tag instead than the macOS version of the build
-   machine.
+   This environment variable is used to specify the target macOS platform major
+   version to the Xcode development tools.  If this environment variable is set,
+   ``meson-python`` will use the specified macOS version for the Python wheel
+   platform tag instead than the macOS version of the build machine.
 
    This variable must be set to macOS major versions only: ``10.9`` to
    ``10.15``, ``11``, ``12``, ``13``, ...