grdgradient -Em not explained in docs

[KristofKoch]

Description of the problem

In the grdgradient documentation the -Em option is not explained. -Es & -Ep are documented as expected.

Check in the docs: https://docs.generic-mapping-tools.org/latest/grdgradient.html#e

When checking the usage message on the CLI I get a proper explanation:

   m: Similar to ESRI's 'hillshade' but faster. Note: azimuth and elevation
      are hardwired to 315 and 45 degrees; if you provide other values they
      will be ignored.