docs(iso): Explain sed insert_n_at_the_end mechanism (#6)

setup.sh

@@ -501,6 +501,21 @@ function in_file_in_array_insert_n_at_the_end () {
     arg_file="${1:?}"
     arg_array="${2:?}"
     arg_string="${3:?}"
+
+    # Look for end of array, insert "${arg_string}" right before
+    #
+    # For following example text we're assuming that:
+    # - "${arg_array}"   equals 'HOOKS'
+    # - "${arg_string}"  equals 'net'
+    #
+    # This:
+    # - Finds a 'HOOKS=' array definition, saves it as \1
+    # - Finds as many non-closing parenthesis characters as possible, so all
+    #   characters until just before the final ')' character of the line and
+    #   saves this as \2
+    # - Finds the closing parenthesis character plus all non-line break
+    #   characters until end of line that come after it and saves this as \3
+    # - Adds a space character and 'net' after \2
     sed -ri \
         -e 's'$'\x1''('"${arg_array}"'=)([^)]*)(\)[^\r\n\f]*)'$'\x1''\1\2 '"${arg_string}"'\3'$'\x1''g' \
         "${arg_file}"