[R] Read about ExecPlan and ExecPlan::ToString

[asfimport]

See the following Jiras:

• https://issues.apache.org/jira/browse/ARROW-14233
• https://issues.apache.org/jira/browse/ARROW-15138
• https://issues.apache.org/jira/browse/ARROW-13785

Reporter: Dragoș Moldovan-Grünfeld / @dragosmg
Assignee: Dragoș Moldovan-Grünfeld / @dragosmg

_{Note: This issue was originally created as ARROW-16957. Please see the migration documentation for further details.}