STYLE: improve IF documentation to cover elseif

Source/cmIfCommand.h

@@ -99,6 +99,11 @@ public:
       "    COMMAND1(ARGS ...)\n"
       "    COMMAND2(ARGS ...)\n"
       "    ...\n"
+      "  ELSEIF(expression2)\n"
+      "    # ELSEIF section.\n"
+      "    COMMAND1(ARGS ...)\n"
+      "    COMMAND2(ARGS ...)\n"
+      "    ...\n"
       "  ELSE(expression)\n"
       "    # ELSE section.\n"
       "    COMMAND1(ARGS ...)\n"
@@ -107,8 +112,9 @@ public:
       "  ENDIF(expression)\n"
       "Evaluates the given expression.  If the result is true, the commands "
       "in the THEN section are invoked.  Otherwise, the commands in the "
-      "ELSE section are invoked.  The ELSE section is optional.  Note that "
-      "the same expression must be given to IF, ELSE, and ENDIF.  Long "
+      "ELSE section are invoked.  The ELSEIF and ELSE sections are "
+      "optional. You may have multiple ELSEIF clauses. Note that "
+      "the same expression must be given to IF, and ENDIF.  Long "
       "expressions can be used and the order or precedence is that the "
       "EXISTS, COMMAND, and DEFINED operators will be evaluated first. "
       "Then any EQUAL, LESS, GREATER, STRLESS, STRGREATER, STREQUAL, MATCHES "