Merge bitcoin/bitcoin#27656: build, doc: Adjust comment after PR27254

3ece0eb build, doc: Adjust comment after PR27254 (Hennadii Stepanov)

Pull request description:

  This is a follow up for bitcoin/bitcoin#27254.

ACKs for top commit:
  TheCharlatan:
    ACK 3ece0eb

Tree-SHA512: 36c627524304f0ea964383488acb9b16d0b553cc9cf3bce7a12aec65a7905b13e4582b7b7ec6d1efa32cd3c623969bc00f805ff31d9e6eec86a614e75796c8ff

Author: fanquake

configure.ac

@@ -1075,7 +1075,7 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <malloc.h>]],
 dnl Check for posix_fallocate
 AC_MSG_CHECKING([for posix_fallocate])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
-                   // same as in src/util/system.cpp
+                   // same as in src/util/fs_helpers.cpp
                    #ifdef __linux__
                    #ifdef _POSIX_C_SOURCE
                    #undef _POSIX_C_SOURCE