Make SkRandom::next[US]Fixed1 private; update documentation for SkRandom::nextSScalar1.

SkRandom is unused in Chromium, Android, Mozilla, and Google3.

SkRandom::nextSScalar1 and LCGRandom::nextSScalar1 appear to me by inspection to potentially return -1, so I updated the documentation to match.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1846773002

Review URL: https://codereview.chromium.org/1846773002
3 files changed