CPack/DEB+RPM: Update documentation on debug symbol package creation

Update documentation to elaborate on the behavior between `.ddeb` creation,
CPACK_STRIP_FILES, and if the files are actually stripped.

Closes: #22438

Help/cpack_gen/deb.rst

@@ -650,11 +650,20 @@ Dbgsym packaging has its own set of variables:
  * Mandatory : NO
  * Default   : OFF
 
+.. note::
+
+ Setting this also strips the ELF files in the generated non-dbgsym package,
+ which results in debuginfo only being available in the dbgsym package.
+
 .. note::
 
  Binaries must contain debug symbols before packaging so use either ``Debug``
  or ``RelWithDebInfo`` for :variable:`CMAKE_BUILD_TYPE` variable value.
 
+ Additionally, if :variable:`CPACK_STRIP_FILES` is set, the files will be stripped before
+ they get to the DEB generator, so will not contain debug symbols and
+ a dbgsym package will not get built. Do not use with :variable:`CPACK_STRIP_FILES`.
+
 Building Debian packages on Windows
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

Help/cpack_gen/rpm.rst

@@ -840,6 +840,10 @@ Debuginfo RPM packaging has its own set of variables:
  Binaries must contain debug symbols before packaging so use either ``Debug``
  or ``RelWithDebInfo`` for :variable:`CMAKE_BUILD_TYPE` variable value.
 
+ Additionally, if :variable:`CPACK_STRIP_FILES` is set, the files will be stripped before
+ they get to the RPM generator, so will not contain debug symbols and
+ a debuginfo package will not get built. Do not use with :variable:`CPACK_STRIP_FILES`.
+
 .. note::
 
  Packages generated from packages without binary files, with binary files but