Remove documentation specific to particular old BDWGC releases

Remove documentation specific to particular old BDWGC releases

* doc/README.Mac: Remove mentioning of particular GC versions; adjust
the information (if needed) to match the current GC functionality.
* doc/README.amiga: Likewise.
* doc/README.arm.cross: Likewise.
* doc/README.autoconf: Likewise.
* doc/README.linux: Likewise.
* doc/README.macros (FINALIZE_ON_DEMAND, JAVA_FINALIZATION): Likewise.
* doc/README.sgi: Likewise.
* doc/README.solaris2 (SOLARIS THREADS): Likewise.
* doc/README.symbian: Likewise.
* doc/README.win32 (Threads): Likewise.
* doc/README.win64: Likewise.
* doc/gcdescr.md (Allocation, Thread support,
Thread-local allocation): Likewise.
* doc/gcinterface.md (GC_INIT): Likewise.
* doc/leak.md: Likewise.
* doc/porting.md (Additional requirements for a basic port,
Thread support): Likewise.
* doc/scale.md (Options for enhanced scalability, Performance):
Likewise.
* doc/simple_example.md (Debugging): Do not mention removed
`--enable-full-debug` option of configure.
* include/gc.h (GC_parallel, GC_dont_gc, GC_use_entire_heap,
GC_set_handle_fork): Remove comment that the behavior has changed in
a particular GC version.
* include/gc_inline.h (GC_MALLOC_WORDS_KIND): Likewise.
* include/gc.h (GC_set_free_space_divisor, GC_warn_proc): Reword
comment about ABI incompatible change in some GC version.
* include/gc.h (GC_get_heap_usage_safe): Remove comment that the API
was introduced in a particular GC version.
* include/gc_cpp.h (UseGC): Likewise.
* os_dep.c [LINUX_STACKBOTTOM && IA64] (GC_get_register_stack_base):
Remove mentioning of a particular GC version (and of an old behavior
on an ancient kernel) in a comment.
* win32_threads.c [!GC_PTHREADS && !GC_NO_THREADS_DISCOVERY]
(GC_DllMain): Likewise.
* OS2_MAKEFILE: Likewise.