Add documentation for helper macros.

doc/ctlseqs.texi

@@ -57,6 +57,7 @@ version.
 
 @menu
 * Overview::                   Brief overview of ctlseqs.
+* Helper Macros::              Helper macros provided by ctlseqs.
 * Control Sequence Matching::  Using ctlseqs for matching control sequences.
 * Control Sequence Reading::   Using ctlseqs for reading control sequences.
 * Tips::                       Tips & hints for using ctlseqs.
@@ -155,6 +156,49 @@ terminal emulators.
 @end itemize
 
 
+@node Helper Macros
+@chapter Helper Macros
+
+A helper macro in ctlseqs is a C preprocessor macro representing a control
+function, which expands to a C string literal.
+
+@cindex List of control function types in ctlseqs helper macros
+The control function can be one of the following three types:
+
+@itemize @bullet
+@item Elements from the C0 or C1 set.
+@item Control Sequences.
+@item Other control functions (such as device control functions).
+@end itemize
+
+Name of a helper macro is the function name with @code{CTLSEQS_} as prefix.
+For a control function other than elements from the C0 or C1 set, the
+corresponding helper macro is a function-like macro which may or may not take
+arguments.
+
+Control sequences listed in the helper macros are primarily exerpted from
+@url{https://invisible-island.net/xterm/ctlseqs/ctlseqs.html, XTerm's manual},
+which may differ across implementations.
+
+As ctlseqs does not currently support 8-bit controls, 2-character 7-bit codes
+from the C1 set are used instead of their 1-character 8-bit representation.
+For example, @code{CTLSEQS_CSI} expands to @code{"\x1b["}.
+
+@cindex Helper macro usage example
+The following code snippet is an example usage of helper macros:
+
+@example
+#include <stdio.h>
+#include <ctlseqs.h>
+int main() @{
+    printf(CTLSEQS_BEL);
+    printf(CTLSEQS_XTVERSION());
+    printf(CTLSEQS_CUP("%d", "%d"), 3, 4);
+    fflush(stdout);
+@}
+@end example
+
+
 @node Control Sequence Matching
 @chapter Control Sequence Matching