org-capture-templates: Document %^{prompt}X %-escape syntax

* lisp/org-capture.el (org-capture-templates):
* doc/org-manual.org (Template expansion): Document the supported
custom prompt syntax.

Link: https://old.reddit.com/r/orgmode/comments/1bfgzsz/format_timestamp_after_capture_by_prompt/

doc/org-manual.org

@@ -8281,6 +8281,12 @@ given here:
   Prompt the user for a value for property {{{var(PROP)}}}.  You may
   specify a default value with =%^{PROP|default}=.
 
+- =%^{PROMPT}X=, X is one of g,G,t,T,u,U,C,L ::
+
+  Prompt the user as in =%^X=, but use the custom prompt string.  You
+  may specify a default value and completions with
+ =%^{PROMPT|default|completion1|completion2|completion3...}X=.
+
 - =%^{PROMPT}= ::
 
   Prompt the user for a string and replace this sequence with it.  You

lisp/org-capture.el

@@ -371,6 +371,10 @@ be replaced with content and expanded:
   %^{prompt}  Prompt the user for a string and replace this sequence with it.
               A default value and a completion table can be specified like this:
               %^{prompt|default|completion2|completion3|...}.
+  %^{prompt}X where X is one of g, G, t, T, u, U, C, or L.
+              Same as %^X (see above), but also supply custom
+              prompt/completions.  Default value and completions as in
+              %^{prompt|default|...}X are allowed.
   %?          After completing the template, position cursor here.
   %\\1 ... %\\N Insert the text entered at the nth %^{prompt}, where N
               is a number, starting from 1.