Fix comment related to pg_cryptohash_error()

One of the comments introduced in b69aba7 was worded a bit weirdly, so
improve it.

Reported-by: Sergey Shinderuk
Discussion: https://postgr.es/m/71b9a5d2-a3bf-83bc-a243-93dcf0bcfb3b@postgrespro.ru
Backpatch-through: 14

src/common/cryptohash.c

@@ -252,7 +252,7 @@ pg_cryptohash_free(pg_cryptohash_ctx *ctx)
 /*
  * pg_cryptohash_error
  *
- * Returns a static string providing errors about an error that
+ * Returns a static string providing details about an error that
  * happened during a computation.
  */
 const char *

src/common/cryptohash_openssl.c

@@ -308,7 +308,7 @@ pg_cryptohash_free(pg_cryptohash_ctx *ctx)
 /*
  * pg_cryptohash_error
  *
- * Returns a static string providing errors about an error that
+ * Returns a static string providing details about an error that
  * happened during a computation.
  */
 const char *