Changing brush size: Update "How to change the size of a brush"

• Remove trivial screenshot of Size slider and link instead to common Tool Options docs.
• Replace <orderedlist> with <itemizedlist> as items are in no particular order.
• Link to Input Controllers docs.
• Do not use <guibutton> or <quote> for things that are not buttons but <guilabel>s.
• Use <keycap> for keyboard keys instead of <quote>.
• Remove <keycombo> from single keyboard keys as that's meant for combinations of input actions.
• Configuring settings is not "programming" (which sounds scary).
• Actions are not displayed "in front of" Events but in their own column.
• Do not "click the small triangle" but "expand" an item - custom themes might for example display a plus/minus box etc instead of a triangle.