[ELF] Clarify the first entry of .got.plt NFC

[MaskRay]

Summary

glibc sysdeps/x86_64/dl-machine.h (and on some other architectures) uses it to compute run-time load address of the shared object.

lld/ELF/Arch/X86_64.cpp

RelExpr X86_64::getRelExpr(RelType type, const Symbol &s,
   default:
     error(getErrorLocation(loc) + "unknown relocation (" + Twine(type) +
           ") against symbol " + toString(s));
     return R_NONE;
   }
 }
 
 void X86_64::writeGotPltHeader(uint8_t *buf) const {
-  // The first entry holds the value of _DYNAMIC. It is not clear why that is
-  // required, but it is documented in the psabi and the glibc dynamic linker
-  // seems to use it (note that this is relevant for linking ld.so, not any
-  // other program).
+  // The first entry holds the link-time address of _DYNAMIC. It is documented
+  // in the psABI and glibc before Aug 2021 used the entry to compute run-time
+  // load address of the shared object (note that this is relevant for linking
+  // ld.so, not any other program).
   write64le(buf, mainPart->dynamic->getVA());
 }
 
 void X86_64::writeGotPlt(uint8_t *buf, const Symbol &s) const {
   // See comments in X86::writeGotPlt.
   write64le(buf, s.getPltVA() + 6);
 }