proj_factors(): accept P to be a projected CRS (fixes #2854)

Updated doc:

    Starting with PROJ 8.2, the P object can be a projected CRS, for example
    instantiated from a EPSG CRS code. The factors computed will be those of the
    map projection implied by the transformation from the base geographic CRS of
    the projected CRS to the projected CRS.

    The input geodetic coordinate lp should be such that lp.lam is the longitude
    in radian, and lp.phi the latitude in radian (thus independently of the
    definition of the base CRS, if P is a projected CRS).

1 files changed, 9 insertions, 0 deletions

diff --git a/docs/source/development/reference/functions.rst b/docs/source/development/reference/functions.rst
index 10bb07d0..bf5bb94b 100644
--- a/docs/source/development/reference/functions.rst
+++ b/docs/source/development/reference/functions.rst
@@ -756,6 +756,15 @@ Various
     distortion and meridian convergence. Depending on the underlying projection
     values will be calculated either numerically (default) or analytically.
 
+    Starting with PROJ 8.2, the P object can be a projected CRS, for example
+    instantiated from a EPSG CRS code. The factors computed will be those of the
+    map projection implied by the transformation from the base geographic CRS of
+    the projected CRS to the projected CRS.
+
+    The input geodetic coordinate lp should be such that lp.lam is the longitude
+    in radian, and lp.phi the latitude in radian (thus independently of the
+    definition of the base CRS, if P is a projected CRS).
+
     The function also calculates the partial derivatives of the given
     coordinate.