Address MaskRay's comment and apply his suggesion

riscv-elf.adoc

@@ -1159,8 +1159,8 @@ specification versions are merged.
 
 === Mapping Symbol
 
-The section can have a mixture of code and data or code with different ISAs;
-mapping symbol is a special symbol class used to describe the boundaries.
+The section can have a mixture of code and data or code with different ISAs.
+A number of symbols, named mapping symbols, describe the boundaries.
 
 [%autowidth]
 |===
@@ -1170,17 +1170,20 @@ mapping symbol is a special symbol class used to describe the boundaries.
 | $x<ISA>     | Start of a sequence of instructions with <ISA> extension.
 |===
 
-The mapping symbol for data(`$d`) means the following region is data.
+The mapping symbol should set the type to `STT_NOTYPE`, binding to `STB_LOCAL`,
+and the size of symbol to zero.
 
-The mapping symbol for instruction(`$x`) means the following region are
-instructions,
+The mapping symbol for data(`$d`) indicates the start of a sequence of data bytes.
+
+The mapping symbol for instruction(`$x`) indicates the start of a sequence of
+instructions.
 and it has an optional ISA string, which means the following code regions are
 using ISA is different than the ISA recorded in the arch attribute;
 the ISA information will used until the next instruction mapping symbol;
 an instruction mapping symbol without ISA string means using ISA configuration
 from ELF attribute.
 
-Format and rule of the optional ISA string are same as Tag_RISCV_arch, must
+Format and rule of the optional ISA string are same as `Tag_RISCV_arch`, must
 having explicit version, more detailed rule please refer to <<Attributes>>.
 
 NOTE: The use case for mapping symbol for instruction(`$x`) with ISA information
@@ -1191,10 +1194,6 @@ attribute is recording for minimal execution environment requirements, so the
 ISA information from arch attribute is not enough for the disassembler to
 disassemble the `rv64gcv` version correctly.
 
-NOTE: For toolchain implementation, the linker is permitted to merge the
-adjacency mapping symbol if they are exactly the same symbol name, and strip
-tools are permitted to strip mapping symbol.
-
 == Linker Relaxation
 
 At link time, when all the memory objects have been resolved, the code sequence