Help: Clarify file(GENERATE) only writes output file at generation phase

Closes: #16556

Help/command/file.rst

@@ -304,6 +304,11 @@ Exactly one ``CONTENT`` or ``INPUT`` option must be given.  A specific
 Generated files are modified on subsequent cmake runs only if their content
 is changed.
 
+Note also that ``file(GENERATE)`` does not create the output file until the
+generation phase. The output file will not yet have been written when the
+``file(GENERATE)`` command returns, it is written only after processing all
+of a project's ``CMakeLists.txt`` files.
+
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