Clarify interaction between params of get_path_collection_extents.

That master_transform is ignored is clear looking at the implementation
in _path.h (the `if (Ntransforms)` branch drops master_transform;
compare with the implementation of point_in_path_collection).  One can
also compare
```
from matplotlib.path import *; from matplotlib.transforms import *
get_path_collection_extents(
    Affine2D().scale(2, 3), [Path([(1, 2), (3, 4)])],
    [], [(0, 0)], IdentityTransform())
get_path_collection_extents(
    Affine2D().scale(2, 3), [Path([(1, 2), (3, 4)])],
    [IdentityTransform()], [(0, 0)], IdentityTransform())
```

Author: anntzer

lib/matplotlib/path.py

@@ -1063,6 +1063,7 @@ def get_path_collection_extents(
         Global transformation applied to all paths.
     paths : list of `Path`
     transforms : list of `~matplotlib.transforms.Affine2DBase`
+        If non-empty, this overrides *master_transform*.
     offsets : (N, 2) array-like
     offset_transform : `~matplotlib.transforms.Affine2DBase`
         Transform applied to the offsets before offsetting the path.