Document extended format specifiers

Add documentation for extended format specifiers (which were introduced
in 68b26ff854) to the man page and the
manual. Add a note about strftime()-style extended formats for
"%(start)" and "%(end)".

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

doc/calcurse.1.txt

@@ -256,6 +256,31 @@ Examples
 *`calcurse -t --format-todo '(%p) %m\n'`*::
   List all todo items and put parentheses around the priority specifiers.
 
+Extended format specifiers
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Extended format specifiers can be used if you want to specify advanced
+formatting options. Extended specifiers are introduced by *%(* and are
+terminated by a closing parenthesis (*)*). The following list includes all
+short specifiers and corresponding long options:
+
+* *s*: *(start)*
+* *S*: *(start:epoch)*
+* *d*: *(duration)*
+* *e*: *(end)*
+* *E*: *(end:epoch)*
+* *m*: *(message)*
+* *n*: *(noteid)*
+* *N*: *(note)*
+* *p*: *(priority)*
+
+The *(start)* and *(end)* specifiers support strftime()-style extended
+formatting options that can be used for fine-grained formatting. Additionally,
+the special formats *epoch* (which is equivalent to *(start:%s)* or *(end:%s)*)
+and *default* (which is mostly equivalent to *(start:%H:%M)* or *(end:%H:%M)*
+but displays *..:..* if the item doesn't start/end at the current day) are
+supported.
+
 Notes
 -----