add help for --injectionSources to MFDv5, CFSv2, HSGCT and --injections for Weave

Trying to follow Karl's recommendations from !744 (closed):

 -shared help string defined in lalpulsar/CWMakeFakeData.c
 -displayed through the regular --help output of each lalapp
 -thanks to Karl Wette for implementation aspects
 -refs https://git.ligo.org/CW/software/lalsuite/issues/37

@karl-wette This currently produces a swig error, so I've cancelled the pipeline proactively:

make[5]: Entering directory '/home/dkeitel/git/lvc/lalsuite/lalpulsar/swig'
  GEN      swiglalpulsar.i
  GEN      swiglal_octave.cpp
../include/lal/CWMakeFakeData.h:87: Warning 451: Setting a const char * variable may leak memory.
make[5]: *** [Makefile:1574: swiglal_octave.cpp] Error 1

Googling, I got as far as knowing I should write "%immutable" somewhere, but I can't figure out the right syntax.

I've set the MR to both "squash commits" and "Allow commits from members who can merge", so you can push a fixup if you'd like, or just give me a hint and assign back if you prefer.

PS: The help text itself is basically your suggestion, with an extended first sentence and some reformatting for readability.