docs: add note

Author: kgryte

lib/node_modules/@stdlib/array/base/mskbinary3d/README.md

@@ -76,6 +76,7 @@ The function accepts the following arguments:
 ## Notes
 
 -   The function assumes that the input and output arrays have the same shape.
+-   An element in an input array is "masked" if the corresponding element in the mask array is non-zero.
 
 </section>