Add a tip about pdf file size in print composer

[Jeroen-GroeneBij]

In section 22.3.4 of the QGIS user manual a Tip would be helpful:

.. tip:: Mind opacity settings for vector layers

When you set a layer wide opacity (or opacity for any vector symbology) to less than 100% it forces a rasterised rendering of those objects during pdf exports. Depending on the size of the layer, this may be a considerable increase in the pdf file size.

See qgis issue 57330
Even for fairly simple maps this could mean a difference between a 2 MB pdf or a 20MB pdf

I don't know how to set up a pull request, so I hope this is enough for someone else to add this to the user manual.