chore(cabal): explain performance related build flags

These flags were added in 9f97147cc3. Adding
a comment to clarify the use.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

postgrest.cabal

@@ -179,6 +179,9 @@ library
   else
     ghc-options: -O2  
     if impl(ghc >= 9.12)
+      -- Makes GHC consider cross-module specialization for polymorphic functions
+      -- without explicitly needing to add INLINE, INLINABLE or SPECIALIZE pragmas.
+      -- Slightly increases the binary size but improves performance considerably.
       ghc-options: -fexpose-overloaded-unfoldings -fspecialise-aggressively
 
   if !os(windows)