update comments in log moudule (#632)

### What problem were solved in this pull request?

Issue Number: close #551

Problem:

- The original comment for `Log::operator<<` incorrectly stated that it
"won't output header information" (like function, file, line, etc.).
- However, the actual implementation calls `Log::out()`, which
internally uses `LOG_HEAD` macro to print full header info (timestamp,
PID/TID, context, function, file, line).

This mismatch can mislead users into thinking `operator<<` only writes
raw message content, potentially causing confusion or misuse.

### What is changed and how it works?

Updated the Doxygen-style comment above `Log::operator<<` template
function in `src/common/log/log.h`.

New comment clearly states:
- `operator<<` **does output standard header info** (time, pid/tid, ctx,
func, file, line).
- If user wants to customize or suppress headers, they should use
`output()` / `out()` methods directly.

### Other information
No code logic changed — only comments correction.

Author: Gris333

src/common/log/log.h

@@ -65,10 +65,11 @@ class Log
   static int init(const string &log_file);
 
   /**
-   * These functions won't output header information such as __FUNCTION__,
-   * The user should control these information
-   * If the header information should be outputed
-   * please use LOG_PANIC, LOG_ERROR ...
+   * Stream-style logging with default level.
+   * It will emit the standard log header (time/pid/tid/context/function/file/line)
+   * via LOG_HEAD before the message, consistent with LOG_* macros.
+   * If you need to customize or suppress headers, use output/out with your own
+   * prefix handling instead of this operator.
    */
   template <class T>
   Log &operator<<(T message);