patch 8.1.1913: not easy to compute the space on the command line

Problem:    Not easy to compute the space on the command line.
Solution:   Add v:echospace. (Daniel Hahler, closes #4732)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 1cafba2..aa27bea 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -1768,6 +1768,13 @@
 <		Note: if another deadly signal is caught when v:dying is one,
 		VimLeave autocommands will not be executed.
 
+					*v:echospace* *echospace-variable*
+v:echospace	Number of screen cells that can be used for an `:echo` message
+		in the last screen line before causing the |hit-enter-prompt|.
+		Depends on 'showcmd', 'ruler' and 'columns'.  You need to
+		check 'cmdheight' for whether there are full-width lines
+		available above the last line.
+
 					*v:errmsg* *errmsg-variable*
 v:errmsg	Last given error message.  It's allowed to set this variable.
 		Example: >