docs(tut): Add cross ref to chapter 6

[VorpalBlade]

This helps when users try to use the docs as a reference to solve their issue at hand (as opposed to reading in order and remembering everything).

Related to discussion #544

[coveralls]

Pull Request Test Coverage Report for Build 9652374114

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.4%) to 42.044%

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Files with Coverage Reduction	New Missed Lines	%
src/token/mod.rs	1	75.0%
src/macros/seq.rs	2	85.71%
<!--	Total:	3		-->

Totals
Change from base Build 9407425121:	0.4%
Covered Lines:	1308
Relevant Lines:	3111

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💛 - Coveralls

[VorpalBlade]

I don't think that "coveralls" report is correct. I haven't changed any of the files it claims have new missed lines...

[VorpalBlade]

Change apparently not wanted at all (seemed like an outright reject rather than suggestions for how to improve the PR) and I don't have time to work on this.

[epage]

To be clear, I gave a first-order approximation of how this could be reworked

For example, we could have something like "With Parser::parse converting PResult into Result for use in our application, let's see how we can polish the quality of the reported error message".