Log Message:
Update tex-pgfplots{,-doc} to 1.3.1.

This fixes PR 43165.

Major changes:
	- added and documented |[xy]bar legend| styles and |[xy]bar interval
	legend| styles
	- renamed 'every mesh legend' to 'mesh legend' for consistency
	- improved 'mesh legend' (rescaled and reshiftet it a bit)
	- Imported all patches to PGF written by Christian Feuersaenger.
	- added |tiny| style
	- numerous bug fixes

See ChangeLog in tex-pgfplots-doc for the complete list of changes.