Document various options and include #warnings in docs

[gatlinnewhouse]

Add documentation (including commented #warnings) to many config options:

• SAFEFETCH_FLOATING_ADAPTIVE_WATERMARK
• SAFEFETCH_PIN_BUDDY_PAGES
• SAFEFETCH_JUST_INTERRUPTS_WHILE_TASK_BLOCKED
• SAFEFETCH_RBTREE_MEM_RANGE
• SAFEFETCH_STATIC_KEYS
• SAFEFETCH_WHITELISTING
• SAFEFETCH_MEASURE_DEFENSE
• SAFEFETCH_MEASURE_MEMORY_CONSUMPTION
• SAFEFETCH_DEBUG
• SAFEFETCH_DEBUG_COLLECT_SAMPLES
• SAFEFETCH_DEBUG_COLLECT_VULNERABILITIES
• SAFEFETCH_DEBUG_LEAKS
• SAFEFETCH_DEBUG_PINNING
• SAFEFETCH_DEBUG_TRACING
• SAFEFETCH_PROTECT_PAGES_READABLE