Update and clarify docstring for _pretty__echo_usage()

src/include-bash/pretty.sh

@@ -465,6 +465,8 @@ _pretty__echo_usage() {
     #  *  command mode (`-c`) prints "commands:" header and
     #     indents next arguments,
     #
+    #  *  indent mode (`-i`) just indents next arguments,
+    #
     #  *  plain mode (`--`) prints empty line (new paragraph)
     #     and turns indentations off.
     #
@@ -474,8 +476,8 @@ _pretty__echo_usage() {
     #
     #     A special case of usage mode is when only single
     #     argument is passed to this function; then instead
-    #     of "usage:" header, this usage pattern is prefixed
-    #     with the same string.
+    #     printing "usage:" header on separate string, it is
+    #     joined with the argument to single line.
     #
     # In order to help avoid (rare) conflict between mkusage()
     # switches and your usage patterns, the very first argument,