Add explanation for the g:fzf_colors setting (#1878)

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

man/man1/fzf.1

@@ -288,8 +288,8 @@ format.
     \fBbg+        \fRBackground (current line)
     \fBgutter     \fRGutter on the left (defaults to \fBbg+\fR)
     \fBhl+        \fRHighlighted substrings (current line)
-    \fBinfo       \fRInfo
-    \fBborder     \fRBorder of the preview window and horizontal separators (\fB--border\fR)
+    \fBinfo       \fRInfo line (match counters)
+    \fBborder     \fRBorder around the window (\fB--border\fR and \fB--preview\fR)
     \fBprompt     \fRPrompt
     \fBpointer    \fRPointer to the current line
     \fBmarker     \fRMulti-select marker