Update docstrings

src/jat.sh.skel

 
 jat__cmd() {
     #
-    # Assert that command $@ returns zero (or other)
+    # Assert that command $@ exits with zero (or other)
     #
     # Usage:
     #
 
 jat__cmp() {
     #
-    # Assert that value $@ returns zero (or other)
+    # Assert comparison of $1 and $3 using operator $2
     #
     # Usage:
     #
 
 jat__eval() {
     #
-    # Assert that command in code $1 returns zero (or other)
+    # Assert that code $1 exits with zero (or other)
     #
     # Usage:
     #
-    #     jat__eval [-s R_ESF] [-S ES_EXPR] [--] CODE
+    #     jat__eval [assert-options] [-s R_ESF] [-S ES_EXPR] [--] CODE
     #
     # Run CODE using eval builtin and if exit status is zero, announce assertion
     # success (PASS), otherwise announce assertion failure (FAIL).