Clarify TLS descriptor calling convention.

Make it clear that vector registers are callee-saved.

riscv-elf.adoc

@@ -975,9 +975,11 @@ Up to 3 `unsigned long` may be stored inline within the descriptor. Dynamic link
 implementations may use this to avoid a separate allocation to store data associated
 with the symbol.
 
-The TLS descriptor `resolver` is called with a special calling convention where all
-registers all callee-saved, except `a0` which is used to pass the argument and `t0`
-which is used as the alternate link register.
+The TLS descriptor `resolver` is called with a special calling convention, specified as follows:
+
+- `a0` is used to pass the argument and return value.
+- `t0` is used as the link register.
+- Any other registers are callee-saved. This includes any vector registers when the vector extension is supported.
 
 Example assembler load and store of a thread local variable `i` using the `%tlsdesc_hi`, `%tlsdesc_lo_add`, `%tlsdesc_lo_load` assembler functions and `.tlsdesc_call` assembler directive. The emitted relocations are in the comments.