more small docs work

and don't cache gaussnoise
more pyvips8 docs

and cleanups of various small doc markup errors
add pbm save

previously vips could load but not save pbm (one bit) images

fix a few bugs in float load and save as well, and improve one-bit load
better random numbers for gaussnoise

fixes a problem with repeated use of gaussnoise
stuff
fix pngsave of grey plus many alpha
fiddle with pyvips8 docstrings
doc polish
tinkering with pyvips8 docs