median: add comments to common.fypp

Author: jvdp1

src/common.fypp

@@ -147,7 +147,20 @@ ${prefix + joinstr.join([line.strip() for line in txt.split("\n")]) + suffix}$
   #:endcall
 #:enddef
 
-
+#!
+#! Generates an array rank suffix for subarrays along a dimension
+#!
+#! Args:
+#!   rank (int): Rank of the original variable
+#!   dim (int): Dimension of the variable
+#!
+#! Returns:
+#!   Array rank suffix string enclosed in braces
+#!
+#! E.g.,
+#!  select_subvector(5, 2)
+#!   -> (j_, :, j___, j____, j_____)
+#!
 #:def select_subvector(rank, idim)
   #:assert rank > 0
   #:call join_lines(joinstr=", ", prefix="(", suffix=")")
@@ -161,6 +174,20 @@ ${prefix + joinstr.join([line.strip() for line in txt.split("\n")]) + suffix}$
   #:endcall
 #:enddef
 
+#!
+#! Generates an array rank suffix for arrays
+#!
+#! Args:
+#!   rank (int): Rank of the original variable
+#!   idim (int): Dimension of the variable
+#!
+#! Returns:
+#!   Array rank suffix string enclosed in braces
+#!
+#! E.g.,
+#!  select_subvector(5, 2)
+#!   -> (j_, j___, j____, j_____)
+#!
 #:def reduce_subvector(rank, idim)
   #:assert rank > 0
   #:if rank > 1
@@ -175,6 +202,4 @@ ${prefix + joinstr.join([line.strip() for line in txt.split("\n")]) + suffix}$
   #:endif
 #:enddef
 
-
-
 #:endmute