The manual already mentions it.
OK?
Index: display.c
===================================================================
RCS file: /cvs/src/usr.bin/top/display.c,v
retrieving revision 1.60
diff -u -p -r1.60 display.c
--- display.c 8 Oct 2019 07:26:59 -0000 1.60
+++ display.c 26 Oct 2019 22:25:46 -0000
@@ -803,7 +803,7 @@ show_help(void)
"\n"
"^L - redraw screen\n"
"<space> - update screen\n"
- "+ - reset any g, p, or u filters\n"
+ "+ - reset any P highlight, g, p, or u filters\n"
"1 - display CPU statistics on a single line\n"
"C - toggle the display of command line arguments\n"
"d count - show `count' displays, then exit\n"
