[DOCS] Warn about the caveats with aliasing aliases

[colings86]

Aliases are just tags on indices and not constructs in their own right. At the moment you can create an alias using aliases by doing the following:

curl -XPOST 'localhost:9200/_aliases' -d '{
    "actions" : [
        { "add" : { "index" : "2015-01-07", "alias" : "today" } }
        { "add" : { "index" : "2015-01-06", "alias" : "yesterday" } }
        { "add" : { "index" : "2015-01-05", "alias" : "3DaysAgo" } }
        { "add" : { "index" : "2015-01-04", "alias" : "4DaysAgo" } }
        { "add" : { "index" : "2015-01-03", "alias" : "5DaysAgo" } }
        { "add" : { "index" : "2015-01-02", "alias" : "6DaysAgo" } }
        { "add" : { "index" : "2015-01-01", "alias" : "7DaysAgo" } }
    ]
}'

curl -XPOST 'localhost:9200/_aliases' -d '{
    "actions" : [
        { "add" : { "index" : "today", "alias" : "thisWeek" } }
        { "add" : { "index" : "yesterday", "alias" : "thisWeek" } }
        { "add" : { "index" : "3DaysAgo", "alias" : "thisWeek" } }
        { "add" : { "index" : "4DaysAgo", "alias" : "thisWeek" } }
        { "add" : { "index" : "5DaysAgo", "alias" : "thisWeek" } }
        { "add" : { "index" : "6DaysAgo", "alias" : "thisWeek" } }
        { "add" : { "index" : "7DaysAgo", "alias" : "thisWeek" } }
    ]
}'

This makes it look like thisWeek points to today which points to 2015-01-07. What actually happens is that 2015-01-07 gets two aliases: today and thisWeek. This works absolutely fine but anyone using this method needs to be aware that if they update the today alias to point to 08-01-2015 it will not update the thisWeek alias and they will need to update that alias as well.

Is this something we should add to the documentation?

[javanna]

yeah tricky bits @colings86 , least we can do is documenting this. I wonder if we should just prevent users from creating aliases against aliases till we properly support it as this seems like a trap.

[javanna]

With #23977, in master we expand index expressions against indices only when managing aliases. I find that this is even better than updating our docs on the caveats of aliasing aliases, as that is not no longer allowed. Closing.