Document adaptive height waiting for the input (#4915)

fzf cannot pick the height until the input ends or exceeds it, so a
stream that stays below the height delays the first render.

man/man1/fzf.1

@@ -408,6 +408,9 @@ Adaptive height has the following limitations:
 * Cannot be used with top/bottom margin and padding given in percent size
 .br
 * It will not find the right size when there are multi-line items
+.br
+* fzf cannot start until the input ends or exceeds the height, so a stream
+that stays below it will delay the first render
 
 .TP
 .BI "\-\-min\-height=" "HEIGHT[+]"