Merge "Doc updates for #apex and #systemapi"

### systemapi

This is a synonym of the `apex` tag. It should be used to clarify that the API

is an API exposed by the system for an APEX, whereas `apex` should be used for

APIs exposed by an APEX to the platform or another APEX.

Indicates that the symbol is exposed by the platform for an apex. Whereas `apex`

should be used for APIs exposed by an APEX to the platform or another APEX.

May be used in combination with `llndk` if the symbol is exposed to both APEX

and the LL-NDK.

Since a single library can be installed ether in platform or an apex, but not

both, a single map.txt file should not contain _both_ # apex and # systemapi symbols.

The granularity between # apex and # systemapi exists to help the API review

process (b/191371676). These two symbols have very similar lifetime "in

practice". A #systemapi symbol can be dropped from the next release if we are

confident that no one is using it. Similarily, #apex can be dropped if we are

sure that the old platform which used the symbol has reached EOL and thus is no

longer accepting new APEX updates. Unlike the APIs for apps where we have zero

control over how APIs are used, we are in a much more controllable environment

when talking about #systemapi and #apex symbols. So, we have some flexibility

here when determining the lifetime of a symbol.

### var