[PAuthABI] Clarify overriding default ELF marking scheme.

The default PAuth vendor and version for ELF files with no ELF
marking is (0,0) which is incompatible with everything. This is
the safest default as there is no information about what, if any,
signing schema is used by that ELF file.

Implementations can change this default in the presence of
supplemental information, such as a linker command line option.
For example there could be the equivalent of the -z force-bti
which declares that all ELF objects without the BTI property are
considered to be compatible.

pauthabielf64/pauthabielf64.rst

@@ -933,8 +933,12 @@ compatibility.
 
 * The absence of any ELF marking means no information on how pointers
   are signed is available for this ELF file. When used in combination
-  with ELF files that contain ELF marking, then the file is assigned
-  the (``platform identifer``, ``version number``) of (0,0).
+  with ELF files that contain ELF marking, then by default the file is
+  assigned the (``platform identifer``, ``version number``) of (0,0).
+  Implementations may use additional information supplementary to the
+  ELF file, such as linker command-line options, to provide an
+  implementation defined `Core Information`_ for ELF files with no ELF
+  marking.
 
 * The presence of ELF marking means that, if the file contains
   pointers, they were signed in a compatible way with the schema
@@ -966,7 +970,7 @@ compatibility.
     Information`_ is not recognized by the loader.
 
   * If the platform identifier from the `Core Information`_ is the
-    reservd Invalid value 0.
+    reserved Invalid value 0.
 
 Platforms may replace the base compatibility model with a
 platform-specific model.