Clarify install target docs.

[hjoliver]

Companion of cylc/cylc-doc#929

Current docs are a bit misleading (IMO) on exactly what an install target is.

Question: can we also change install target = localhost in various example to something else, e.g. local_cluster to make it more obvious that an install target is not a hostname?

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

Base branch should be 8.6.x, also lint failing currently.

Also I would be tempted to also do this to avoid confusion:

diff --git a/etc/conf/global.cylc b/etc/conf/global.cylc
index d02870936..14e644846 100644
--- a/etc/conf/global.cylc
+++ b/etc/conf/global.cylc
@@ -9,3 +9,3 @@
         hosts = _remote_background_indep_tcp
-        install target = _remote_background_indep_tcp
+        install target = indep
     #[[_remote_background_indep_ssh]]
@@ -14,3 +14,3 @@
     #    hosts = _remote_background_indep_tcp
-    #    install target = _remote_background_indep_tcp
+    #    install target = indep
     #    communication method = ssh
@@ -20,6 +20,6 @@
         hosts = _remote_background_indep_tcp
-        install target = _remote_background_indep_tcp
+        install target = indep
     [[_remote_background_indep_poll]]
         hosts = _remote_background_indep_poll
-        install target = _remote_background_indep_poll
+        install target = indep
         communication method = poll
@@ -31,3 +31,3 @@
         hosts = _remote_background_indep_poll
-        install target = _remote_background_indep_poll
+        install target = indep
         communication method = poll
@@ -37,2 +37,3 @@
     #    hosts = _remote_background_shared_tcp
+    #     install target = localhost
     #[[_remote_background_shared_ssh]]
@@ -42,2 +43,3 @@
     #    communication method = ssh
+    #     install target = localhost
     #[[_remote_background_shared_poll]]
@@ -47 +49,2 @@
     #    submission polling intervals = PT1S
+    #     install target = localhost

[hjoliver]

(Leaving this alone until questions on the main doc PR are resolved)