Pale Moon system-* flag descriptions

[James-E-A]

For general quality control we probably ought to add descriptions for the missing flags:

~ equery u palemoon      
[ Legend : U - final flag setting for installation]
[        : I - package is installed with flag     ]
[ Colors : set, unset                             ]
 * Found these USE flags for www-client/palemoon-27.2.1-r1:
 U I
 - - alsa               : Add support for media-libs/alsa-lib (Advanced Linux Sound Architecture)
 - - bindist            : Flag to enable or disable options for prebuilt (GRP) packages (eg. due to licensing issues)
 + + cpu_flags_x86_sse2 : Use the SSE2 instruction set
 + + cups               : Add support for CUPS (Common Unix Printing System)
 + + custom-cflags      : Build with user-specified CFLAGS (unsupported)
 + + dbus               : Enable dbus support for anything that needs it (gpsd, gnomemeeting, etc)
 + + devtools           : <unknown>
 - - disable-optimize   : <unknown>
 + + ffmpeg             : Enable ffmpeg/libav-based audio/video codec support
 + + gold               : <unknown>
 + + jemalloc           : <unknown>
 + + pulseaudio         : Add support for PulseAudio sound server
 + + system-bz2         : <unknown>
 + + system-cairo       : <unknown>
 + + system-ffi         : <unknown>
 + + system-icu         : <unknown>
 + + system-jpeg        : <unknown>
 + + system-libevent    : <unknown>
 + + system-nspr        : <unknown>
 + + system-nss         : <unknown>
 + + system-pixman      : <unknown>
 + + system-png         : <unknown>
 + + system-spell       : <unknown>
 + + system-sqlite      : <unknown>
 + + system-vpx         : <unknown>
 + + system-webp        : <unknown>
 + + system-zlib        : <unknown>
 + + threads            : Add threads support for various packages. Usually pthreads

[James-E-A]

Fixed with 7da170699ae1e567492eb88e55ae2e4005f81b6a