docs/manual/pkg-cmake.txt: add _INSTALL_OPTS description

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/adding-packages-cmake.txt

@@ -126,6 +126,11 @@ typical packages will therefore only use a few of them.
   in the build step. These are passed after the +make+ command. By
   default, empty.
 
+* +LIBFOO_INSTALL_OPTS+ contains the make options used to
+  install the package to the host directory. By default, the value
+  is +install+, which is correct for most CMake packages. It is still
+  possible to override it.
+
 * +LIBFOO_INSTALL_STAGING_OPTS+ contains the make options used to
   install the package to the staging directory. By default, the value
   is +DESTDIR=$(STAGING_DIR) install/fast+, which is correct for most