[DOCS] Rephrase built-in watcher_admin role's description

[pickypg]

watcher_admin
Grants read access to the .watches index, read access to the watch history and the triggered watches index and allows to execute all watcher actions.

I was sure that this role acted as the admin (it does), but it starts by talking about read privileges. Can we swap the sentence around so that the important bit is at the front? Something like:

Allows users to create and execute all watcher actions. Grants read access to the .watches index, read access to the watch history and the triggered watches index.

Relates to elastic/stack-docs#190

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)