Improve theme documentation

[paleolimbot]

There is an opportunity to improve the documentation related to themes, including the complete theme documentation, the theme() function documentation, and the default themes documentation. I seem to remember @clauswilke is in the process of improving themes in general, but I think these issues are general enough to tackle separately. This is a documentation issue intended for the tidyverse dev day (#3267).

• The difference between theme_update() and theme_replace() is not clear from the documentation in theme_get (Updating a theme to remove panel.grid.major.x #3019). There are examples that make this distinction but not any text. This page should also link to theme(), which it currently does not.

• theme() documentation should link to the complete themes (i.e., ggthemes) somewhere in the first paragraph (as the documentation for the complete themes does for theme()).

[clauswilke]

Yes, I plan to work on the theme implementation, but that shouldn't prevent improvements to the current docs. Most of my work will be under the hood and won't affect the main usage patterns of themes.