There is no need to show two headers when we are only displaying a single
section of search results, so we merge the section header with the main
header in that case.
Show a back button on the left of the main header to get back to the results
for all sections.
Remove this._showTooltip flag from SearchSectionHeader because we no longer
show this type of header when we want to suppress the tooltip.
Add this._appSearchResultsOnlyShown and this._docSearchResultsOnlyShown
flags to track when a single section is shown more cleanly.
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