Update customise.Rmd to advice for search on the right + note about pkgdown changing default navbar (#2710)

---------

Co-authored-by: Hadley Wickham <[email protected]>

Author: olivroy

vignettes/customise.Rmd

@@ -282,6 +282,8 @@ It makes use of the the following built-in components:
 -   `github`, a link to the source repository (with an icon), if it can be automatically determined from the `DESCRIPTION`.
 -   `lightswitch`, a ["light switch"](#light-switch) to select light mode, dark mode, or auto mode.
 
+Note that customising `navbar` like this comes with a downside: if pkgdown later changes the defaults, you'll have to update your `_pkgdown.yml`.
+
 [^dots]: Note that dots (`.`) in the package name need to be replaced by hyphens (`-`) in the vignette filename to be recognized as the intro. That means for a
     package `foo.bar` the intro needs to be named `foo-bar.Rmd`.
 
@@ -333,7 +335,7 @@ For example, the following yaml adds a new "twitter" component that appears to t
 ``` yaml
 navbar:
   structure:
-    right: [twitter, github]
+    right: [search, twitter, github, lightswitch]
   components:
     twitter:
       icon: fa-twitter