Docs: Document the "j" key in the agenda

doc/org.texi

@@ -6768,6 +6768,10 @@ Display the previous dates.
 @item .
 Go to today.
 @c
+@kindex j
+@item j
+Prompt for a date and go there.
+@c
 @kindex C-c C-x C-c
 @item C-c C-x C-c
 @vindex org-columns-default-format

doc/orgcard.tex

@@ -642,7 +642,7 @@ after  ``{\tt :}'', and dictionary words elsewhere.
 \key{filter with repect to a tag}{/}
 \key{save all org-mode buffers}{s}
 \key{display next/previous day,week,...}{RIGHT/LEFT}
-\key{goto today}{.}
+\key{goto today / some date (prompt)}{. / j}
 
 {\bf Remote editing}