langref: mark unsafe builtin functions

[perillo]

Some builtin functions can cause undefined behavior or subvert the Zig type system.
I think these builtin functions should be clearly marked.

Here is a list, probably incomplete:

• @addrSpaceCast
  The documentation says that the operation may be illegal. What is the difference from undefined behavior?
  There is no documentation in the "Undefined Behavior" section.
• @ptrCast
  The documentation says:
  "Casting an optional pointer which is null to a non-optional pointer invokes safety-checked Undefined Behavior".
  An additional warning is in the "volatile" section:
  "To convert one pointer type to another, use @ptrCast. This is an unsafe operation that Zig cannot protect you against."
  This is also documented in the "Undefined Behavior" section.
• @alignCast
  Incorrect alignment is also documented in "Undefined Behavior" section.
• @qualCast
  Warning about unsafe behavior is missing.
  Undefined behavior caused by @qualCast should probably be added to the "Undefined Behavior".

[rohlem]

Potential sub-task of #1966 I think - marking every enumerated operation that it may be illegal probably makes sense for all of them.

[perillo]

Potential sub-task of #1966 I think - having back-links to note on each enumerated operation that it may be illegal probably makes sense in general.

Thanks. I have reported the new UB caused by @qualCast in that issue.