Reword comment and NEWS entry.

Author: gpshead

Lib/base64.py

@@ -602,7 +602,7 @@ def main():
             func(f, sys.stdout.buffer)
     else:
         if sys.stdin.isatty():
-            # gh-138775: read input data at once when reading from stdin.
+            # gh-138775: read terminal input data all at once to detect EOF
             import io
             data = sys.stdin.buffer.read()
             buffer = io.BytesIO(data)

Misc/NEWS.d/next/Library/2025-09-11-15-03-37.gh-issue-138775.w7rnSx.rst

@@ -1 +1,2 @@
-:mod:`base64`: fix the EOF being ignored in command-line interface.
+Use of ``python -m`` with :mod:`base64` has been fixed to detect input from a
+terminal so that it properly notices EOF.