Add initial documentation framework for TypEr.
Prepare release
Correct the name of a compiler option and fix the corresponding warnings

The option `warn_export_vars` was erroneously written as `warn_exported_vars`
in Makefiles of dialyzer and typer. As a result, `erlc` was silent about such
warnings (why on earth doesn't it complain on unrecognized options??) which
were corrected as part of this change.
Merge branch 'kostis/dialyzer-warn_export_vars' into maint

* kostis/dialyzer-warn_export_vars:
  Correct the name of a compiler option and fix the corresponding warnings
Introduce the attribute -optional_callbacks in the context of behaviours
Prepare release
Merge tag 'OTP-17.1'

=== OTP-17.1 ===

Changed Applications:
- asn1-3.0.1
- common_test-1.8.1
- compiler-5.0.1
- crypto-3.4
- debugger-4.0.1
- dialyzer-2.7.1
- diameter-1.7
- erl_interface-3.7.17
- erts-6.1
- hipe-3.11
- inets-5.10.2
- kernel-3.0.1
- mnesia-4.12.1
- observer-2.0.1
- reltool-0.6.6
- ssh-3.0.3
- ssl-5.3.5
- stdlib-2.1
- syntax_tools-1.6.15
- test_server-3.7.1
- tools-2.6.15
- typer-0.9.8
- wx-1.3

Unchanged Applications:
- cosEvent-2.1.15
- cosEventDomain-1.1.14
- cosFileTransfer-1.1.16
- cosNotification-1.1.21
- cosProperty-1.1.17
- cosTime-1.1.14
- cosTransactions-1.2.14
- edoc-0.7.13
- eldap-1.0.3
- erl_docgen-0.3.5
- et-1.5
- eunit-2.2.7
- gs-1.5.16
- ic-4.3.5
- jinterface-1.5.9
- megaco-3.17.1
- odbc-2.10.20
- orber-3.6.27
- os_mon-2.2.15
- ose-1.0
- otp_mibs-1.0.9
- parsetools-2.0.11
- percept-0.8.9
- public_key-0.22
- runtime_tools-1.8.14
- sasl-2.4
- snmp-4.25.1
- webtool-0.8.10
- xmerl-1.3.7