Clarify principal_axes() axis orientation in documentation (#1828)

Author: Akshit Boora

package/MDAnalysis/core/topologyattrs.py

@@ -2073,7 +2073,12 @@ def principal_axes(group, wrap=False):
         axis_vectors : array
             3 x 3 array with ``v[0]`` as first, ``v[1]`` as second, and
             ``v[2]`` as third eigenvector.
-
+        Notes
+	-----
+	The returned array contains the principal axes as row vectors.
+	Each row corresponds to one principal axis (e1, e2, e3). This may
+	appear transposed compared to conventions where eigenvectors are
+	stored as column vectors.
 
         .. versionchanged:: 0.8
            Added `pbc` keyword