Clarify <nf> in asm syntax equals NFIELDS (riscv#1986)

Resolves riscv#1983

Author: aswaterman

src/v-st-ext.adoc

@@ -1838,6 +1838,7 @@ segment loads and stores respectively.
 
 ----
 # Format
+# In this syntax, <nf> equals NFIELDS and is an integer in the range [2, 8].
 vlseg<nf>e<eew>.v vd, (rs1), vm      # Unit-stride segment load template
 vsseg<nf>e<eew>.v vs3, (rs1), vm     # Unit-stride segment store template