[ELF] - Added additional comments on top of r292789 (D29021)

It was requested during post commit review.

llvm-svn: 292903

lld/ELF/InputSection.cpp

@@ -237,6 +237,10 @@ template <class RelTy>
 void InputSection<ELFT>::copyRelocations(uint8_t *Buf, ArrayRef<RelTy> Rels) {
   InputSectionBase<ELFT> *RelocatedSection = getRelocatedSection();
 
+  // Loop is slow and have complexity O(N*M), where N - amount of
+  // relocations and M - amount of symbols in symbol table.
+  // That happens because getSymbolIndex(...) call below performs
+  // simple linear search.
   for (const RelTy &Rel : Rels) {
     uint32_t Type = Rel.getType(Config->Mips64EL);
     SymbolBody &Body = this->File->getRelocTargetSym(Rel);