Document USE_PDF_LATEXMK feature flag

Author: humitos

docs/guides/feature-flags.rst

@@ -12,6 +12,8 @@ or disable one or more of these featured flags for a particular project.
 Available Flags
 ---------------
 
+``USE_PDF_LATEXMK``: :featureflags:`USE_PDF_LATEXMK`
+
 ``USE_SPHINX_LATEST``: :featureflags:`USE_SPHINX_LATEST`
 
 ``USE_SETUPTOOLS_LATEST``: :featureflags:`USE_SETUPTOOLS_LATEST`