[SIL] Update docs to always use '@convention'
[SIL] Update docs to always use '@convention'

Updates the SIL documentation to use the @convention attribute on function
types. The @thin and @cc attributes are obsolete and result in errors when
used in SIL code.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift