Clarify Tag_RISCV_arch is the minimum execution environment requireme…

…nt of objects

And also add NOTE to mention we could use more than Tag_RISCV_arch
specify, but must make sure that should work on the execution
environment with what Tag_RISCV_arch specify only.

riscv-elf.adoc

@@ -954,9 +954,9 @@ default value is 16 for RV32I or RV64I, and 4 for RV32E.
 It will report erros if link object files with different Tag_RISCV_stack_align values.
 
 ===== Tag_RISCV_arch, 5, NTBS=subarch
-Tag_RISCV_arch contains a string for the target architecture taken from
-the option `-march`. Different architectures will be integrated into a superset
-when object files are merged.
+Tag_RISCV_arch contains a string for the target architecture, this information
+used as the minimum execution environment requirement. Different architectures
+will be integrated into a superset when object files are merged.
 
 Note that the version information for target architecture must be presented
 explicitly in the attribute and abbreviations must be expanded. The version
@@ -967,6 +967,10 @@ its based ISA. On the other hand, the architecture `RV32G` has to be presented
 as `RV32I2P0_M2P0_A2P0_F2P0_D2P0` in which the abbreviation `G` is expanded
 to the IMAFD combination with default versions of the standard extensions.
 
+NOTE: Object files could use instruction extensions beyond than what
+Tag_RISCV_arch specified, but object files should make sure that they can
+safely run on the execution environment without those instruction extensions.
+
 ===== Tag_RISCV_unaligned_access, 6, uleb128=value
 Tag_RISCV_unaligned_access denotes the code generation policy for this object
 file. Its values are defined as follows: