util.h: Document FileCommit function

Author: eklitzke

src/util/system.h

@@ -56,6 +56,11 @@ bool error(const char* fmt, const Args&... args)
 }
 
 void PrintExceptionContinue(const std::exception *pex, const char* pszThread);
+
+/**
+ * Ensure file contents are fully committed to disk, using a platform-specific
+ * feature analogous to fsync().
+ */
 bool FileCommit(FILE *file);
 bool TruncateFile(FILE *file, unsigned int length);
 int RaiseFileDescriptorLimit(int nMinFD);