patch 8.1.1275: cannot navigate to errors before/after the cursor

Problem:    Cannot navigate to errors before/after the cursor.
Solution:   Add the :cbefore and :cafter commands. (Yegappan Lakshmanan,
            closes #4340)
diff --git a/runtime/doc/index.txt b/runtime/doc/index.txt
index 7192905..9b5663e 100644
--- a/runtime/doc/index.txt
+++ b/runtime/doc/index.txt
@@ -1192,9 +1192,11 @@
 |:caddbuffer|	:cad[dbuffer]	add errors from buffer
 |:caddexpr|	:cadde[xpr]	add errors from expr
 |:caddfile|	:caddf[ile]	add error message to current quickfix list
+|:cafter|	:caf[ter]	go to error after current cursor
 |:call|		:cal[l]		call a function
 |:catch|	:cat[ch]	part of a :try command
-|:cbelow|	:cbe[low]	got to error below current line
+|:cbefore|	:cbef[ore]	go to error before current cursor
+|:cbelow|	:cbel[ow]	go to error below current line
 |:cbottom|	:cbo[ttom]	scroll to the bottom of the quickfix window
 |:cbuffer|	:cb[uffer]	parse error messages and jump to first error
 |:cc|		:cc		go to specific error
@@ -1356,10 +1358,12 @@
 |:laddexpr|	:lad[dexpr]	add locations from expr
 |:laddbuffer|	:laddb[uffer]	add locations from buffer
 |:laddfile|	:laddf[ile]	add locations to current location list
+|:lafter|	:laf[ter]	go to location after current cursor
 |:last|		:la[st]		go to the last file in the argument list
 |:language|	:lan[guage]	set the language (locale)
 |:later|	:lat[er]	go to newer change, redo
-|:lbelow|	:lbe[low]	go to location below current line
+|:lbefore|	:lbef[ore]	go to location before current cursor
+|:lbelow|	:lbel[ow]	go to location below current line
 |:lbottom|	:lbo[ttom]	scroll to the bottom of the location window
 |:lbuffer|	:lb[uffer]	parse locations and jump to first location
 |:lcd|		:lc[d]		change directory locally