Expand acronyms of C.ADDI16SP/C.ADDI4SPN (riscv#1619)

Resolves riscv#1618

src/c-st-ext.adoc

@@ -605,7 +605,8 @@ zero for C.ADDIW, where this corresponds to `sext.w rd`. C.ADDIW is
 only valid when `_rd_≠x0`; the code points with
 `_rd_=x0` are reserved.
 
-C.ADDI16SP shares the opcode with C.LUI, but has a destination field of
+C.ADDI16SP (add immediate to stack pointer)
+shares the opcode with C.LUI, but has a destination field of
 `x2`. C.ADDI16SP adds the non-zero sign-extended 6-bit immediate to the
 value in the stack pointer (`sp=x2`), where the immediate is scaled to
 represent multiples of 16 in the range (-512,496). C.ADDI16SP is used to
@@ -623,7 +624,8 @@ always 16-byte aligned.
 include::images/wavedrom/c-ciw.edn[]
 //.CIW format.
 (((compressed, CIW)))
-C.ADDI4SPN is a CIW-format instruction that adds a _zero_-extended
+C.ADDI4SPN (add immediate to stack pointer, non-destructive)
+is a CIW-format instruction that adds a _zero_-extended
 non-zero immediate, scaled by 4, to the stack pointer, `x2`, and writes
 the result to `rd′`. This instruction is used to generate
 pointers to stack-allocated variables, and expands to