docs(README): Suggest using completionsdir as a fallback

[jtojnar]

If the bash-completion.pc file is not available during build,
the build system would fall back to using the legacy compatdir.

Let’s update the fallback to make it consistent with the optimistic branch.

[scop]

Makes sense to me, but in the same answer we later refer to datarootdir instead of datadir. I've read the descriptions of the two, became confused, but was left with the feeling that datadir would actually be what we should be using (mostly because the datadir description mentions it actually being the dir for installing things into, datarootdir does not). We also use datadir for the replacements we do for the .pc file, not datarootdir.

This is not the fault of this change, but as it makes the difference so obvious, I'd like to discuss here what to do about it. I guess I'm suggesting this (to be done in another commit/PR):

--- a/README.md
+++ b/README.md
@@ -236 +236 @@ A. [ Disclaimer: Here, how to make the completion code visible to
-   bashcompdir = $(datarootdir)/bash-completion/completions
+   bashcompdir = $(datadir)/bash-completion/completions
@@ -243 +243 @@ A. [ Disclaimer: Here, how to make the completion code visible to
-   install(FILES your-completion-file DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/bash-completion/completions")
+   install(FILES your-completion-file DESTINATION "${CMAKE_INSTALL_DATADIR}/bash-completion/completions")

@akinomyoga your thoughts on this? Based on git log you were involved with the commit that added these two mentions, 530017d