Lacking documentation of colored-completion-prefix

[Michal Pesa]

Hello,
documentation for the readline variable "colored-completion-prefix" 
doesn't state which part of the "LS_COLORS" is used.

Thank you for your time.

Configuration Information:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' 
-DSHELL -DHAVE_$
uname output: Linux pish-lub 4.15.0-29-generic #31-Ubuntu SMP Tue Jul 17 
15:39:52 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.4
Patch Level: 19
Release Status: release