Rework Nonstandard Relocations

The primary change here is to require that vendor identifier symbols are
Defined, STB_LOCAL, STT_NOTYPE, and zero-sized.

This also slightly reorganises what was the "Vendor Identifiers" section
to bring together all the paragraphs about the nonstandard relocations,
so they are in one place. This rewording is not intended to change the
meaning of what was written before.

Author: lenary

riscv-elf.adoc

@@ -556,15 +556,6 @@ Description:: Additional information about the relocation
                                             <|
 |===
 
-Nonstandard extensions are free to use relocation numbers 192-255 for any
-purpose. These vendor-specific relocations must be preceded by a
-`R_RISCV_VENDOR` relocation against a vendor ID symbol.
-
-Where possible, tools should present relocation as their vendor-specific
-relocation types, otherwise a generic name of `R_RISCV_CUSTOM<enum value>` must
-be shown. Data contained in paired `RISCV_VENDOR` can be used to select the
-appropriate vendor when performing relocations.
-
 This section and later ones contain fragments written in assembler. The precise
 assembler syntax, including that of the relocations, is described in the
 _RISC-V Assembly Programmer's Manual_ <<rv-asm>>.
@@ -576,15 +567,27 @@ and fill the space with a single ULEB128-encoded value.
 This is achieved by prepending the redundant `0x80` byte as necessary.
 The linker must not alter the length of the ULEB128-encoded value.
 
-==== Vendor identifiers
+==== Nonstandard Relocations (a.k.a. Vendor-Specific Relocations)
 
-Vendor identifiers are dummy symbols used in the corresponding `R_RISCV_VENDOR`
-relocation (irrespective of ELF class/XLEN) and must be unique amongst all
-vendors providing custom relocations. Vendor identifiers may be suffixed with a
-tag to provide extra relocations for a given vendor.
+Nonstandard extensions are free to use relocation numbers 192-255 for any
+purpose. These vendor-specific relocations must be preceded by a
+`R_RISCV_VENDOR` relocation against a vendor identifier symbol. The preceding
+`R_RISCV_VENDOR` relocation is used by the linker to choose the correct
+implementation for the associated nonstandard relocation.
+
+The vendor identifier symbol should be a defined symbol and should set the type
+to `STT_NOTYPE`, binding to `STB_LOCAL`, and the size of symbol to zero.
+
+Vendor identifiers must be unique amongst all vendors providing custom
+relocations. Vendor identifiers may be suffixed with a tag to provide extra
+relocations for a given vendor.
 
 NOTE: Please refer to the _RISC-V Toolchain Conventions_
-<<rv-toolchain-conventions>> for the full list.
+<<rv-toolchain-conventions>> for the full list of vendor identifiers.
+
+Where possible, tools should present relocation as their vendor-specific
+relocation types, otherwise a generic name of `R_RISCV_CUSTOM<enum value>` must
+be shown.
 
 ==== Calculation Symbols