Add key and documentation for `org-copy-visible'.

* lisp/org.el (org-org-menu): Add `org-copy-visible' to the menu.
* doc/org.texi (Visibility cycling): Document `org-copy-visible'.
* doc/orgcard.tex: Document `org-copy-visible'.

doc/org.texi

@@ -1216,6 +1216,8 @@ but without affecting visibility in that buffer.}.  With a numeric
 prefix argument N, go up to level N and then take that tree.  If N is
 negative then go up that many levels.  With a @kbd{C-u} prefix, do not remove
 the previously used indirect buffer.
+@orgcmd{C-c C-x v,org-copy-visible}
+Copy the @i{visible} text in the region into the kill ring.
 @end table
 
 @vindex org-startup-folded

doc/orgcard.tex

@@ -309,6 +309,7 @@ are preserved on all copies.
 \metax{move subtree/list item up/down}{M-S-UP/DOWN}
 \metax{sort subtree/region/plain-list}{C-c \^{}}
 \metax{clone a subtree}{C-c C-x c}
+\metax{copy visible text}{C-c C-x v}
 \metax{kill/copy subtree}{C-c C-x C-w/M-w}
 \metax{yank subtree}{C-c C-x C-y or C-y}
 \metax{narrow buffer to subtree / widen}{C-x n s/w}

lisp/org.el

@@ -16810,6 +16810,7 @@ BEG and END default to the buffer boundaries."
 (org-defkey org-mode-map "\C-c\C-m" 'org-ctrl-c-ret)
 (org-defkey org-mode-map "\M-\C-m"  'org-insert-heading)
 (org-defkey org-mode-map "\C-c\C-xc" 'org-clone-subtree-with-time-shift)
+(org-defkey org-mode-map "\C-c\C-xv" 'org-copy-visible)
 (org-defkey org-mode-map [(control return)] 'org-insert-heading-respect-content)
 (org-defkey org-mode-map [(shift control return)] 'org-insert-todo-heading-respect-content)
 (org-defkey org-mode-map "\C-c\C-x\C-n" 'org-next-link)
@@ -18298,6 +18299,8 @@ See the individual commands for more information."
      "--"
      ["Clone subtree, shift time" org-clone-subtree-with-time-shift t]
      "--"
+     ["Copy visible text"  org-copy-visible t]
+     "--"
      ["Promote Heading" org-metaleft (not (org-at-table-p))]
      ["Promote Subtree" org-shiftmetaleft (not (org-at-table-p))]
      ["Demote Heading"  org-metaright (not (org-at-table-p))]