Build system: reduce verbosity of documentation commands

Cmake will print the entire command line for custom targets if no
COMMENT field is set. Add that to reduce the verbosity and make the
output a bit more manageable; verbose builds will still print the entire
command.

Change-Id: Ifedcb75fed4614788ffa9a6b121e2b435810a379
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

cmake/QtDocsHelpers.cmake

@@ -208,6 +208,7 @@ function(qt_internal_add_docs)
         --basedir "${PROJECT_SOURCE_DIR}/.."
         --filter "QDocModule=${doc_target}"
         -o "${target_bin_dir}/codeattributions.qdoc"
+        COMMENT "Scanning attributions for ${target}..."
     )
 
     # prepare docs target
@@ -269,6 +270,7 @@ function(qt_internal_add_docs)
         COMMAND ${CMAKE_COMMAND} -E env ${qdoc_env_args}
         ${qdoc_bin}
         ${prepare_qdoc_args}
+        COMMENT "Running qdoc for ${target}..."
     )
 
     add_dependencies(prepare_docs_${target} qattributionsscanner_${target})
@@ -337,6 +339,7 @@ function(qt_internal_add_docs)
             COMMAND ${qhelpgenerator_bin}
             "${qdoc_output_dir}/${doc_target}.qhp"
             -o "${qch_file_path}"
+            COMMENT "Building QtHelp files for ${target}..."
         )
     endforeach()
     add_dependencies(qch_docs_${target} generate_docs_${target})