patch 8.2.2508: cannot change the character displayed in non existing lines

Problem:    Cannot change the character displayed in non existing lines.
Solution:   Add the "eob" item to 'fillchars'. (closes #7832, closes #3820)
diff --git a/runtime/doc/windows.txt b/runtime/doc/windows.txt
index 2a31526..69de9ba 100644
--- a/runtime/doc/windows.txt
+++ b/runtime/doc/windows.txt
@@ -138,10 +138,12 @@
 that have termcap codes for italics.
 
 							*filler-lines*
-The lines after the last buffer line in a window are called filler lines.
-These lines start with a tilde (~) character. By default, these are
-highlighted as NonText (|hl-NonText|). The EndOfBuffer highlight group
-(|hl-EndOfBuffer|) can be used to change the highlighting of filler lines.
+The lines after the last buffer line in a window are called filler lines.  By
+default, these lines start with a tilde (~) character. The 'eob' item in the
+'fillchars' option can be used to change this character. By default, these
+characters are highlighted as NonText (|hl-NonText|). The EndOfBuffer
+highlight group (|hl-EndOfBuffer|) can be used to change the highlighting of
+the filler characters.
 
 ==============================================================================
 3. Opening and closing a window				*opening-window* *E36*