Document that logger now encodes table arguments in JSON

[TarantoolBot]

How it used to work:

tarantool> require('log').info('%s', {foo = 'bar'})
table: 0x41f9d2e8

How it works now:

tarantool> require('log').info('%s', {foo = 'bar'})
{"foo":"bar"}

Note, JSON encoding is enabled only for tables that don't have
the __tostring meta-method:

tarantool> require('log').info('%s', setmetatable({foo = 'bar'}, {__tostring = function() return 'foobar' end}))
foobar

Requested by @locker in tarantool/tarantool@3db73c6.