Correct and unify descriptions of normal mode C key behavior

Fixes #4747

README.asciidoc

@@ -635,8 +635,8 @@ use `n` followed by `,`, in order to remove a selection, use `<a-,>`.
 contains a match for this regex. Using `<a-K>` you can keep the selections
 not containing a match.
 
-`C` copies the current selection to the next line (or lines if a count is given)
-`<a-C>` does the same to previous lines.
+`C` duplicates selections on the lines that follow them.
+`<a-C>` does the same but on the preceding lines.
 
 `$` allows you to enter a shell command and pipe each selection to it.
 Selections whose shell command returns 0 will be kept, other will be dropped.

doc/pages/keys.asciidoc

@@ -570,10 +570,10 @@ to skim through the jump list using:
     select first and last characters of each selection
 
 *C*::
-    copy the main selection to the next line
+    duplicate selections on the lines that follow them
 
 *<a-C>*::
-    copy the main selection to the previous line
+    duplicate selections on the lines that precede them
 
 *,*::
     clear selections to only keep the main one

src/normal.cc

@@ -2364,8 +2364,8 @@ static constexpr HashMap<Key, NormalCmd, MemoryDomain::Undefined, KeymapBackend>
 
     { {'_'}, {"trim selections", trim_selections} },
 
-    { {'C'}, {"copy selection on next lines", copy_selections_on_next_lines<Forward>} },
-    { {alt('C')}, {"copy selection on previous lines", copy_selections_on_next_lines<Backward>} },
+    { {'C'}, {"duplicate selections on the lines that follow them.", copy_selections_on_next_lines<Forward>} },
+    { {alt('C')}, {"duplicate selections on the lines that precede them.", copy_selections_on_next_lines<Backward>} },
 
     { {Key::Space}, {"user mappings", exec_user_mappings} },