[DOCS] Merge payable FAQ item

[ChrisChinchilla]

Merge payable FAQ item into docs, part of #1219

Checklist

• Code compiles correctly
• All tests are passing
• New tests have been created which fail without the change (if possible)
• README / documentation was extended, if necessary
• Changelog entry (if change is visible to the user)
• Used meaningful commit messages

[codecov]

Codecov Report

Merging #5796 into develop will not change coverage.
The diff coverage is n/a.

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##           develop    #5796   +/-   ##
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  Coverage    88.35%   88.35%           
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  Files          348      348           
  Lines        33378    33378           
  Branches      3998     3998           
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  Hits         29492    29492           
  Misses        2532     2532           
  Partials      1354     1354

Flag	Coverage Δ
#all	88.35% <ø> (ø)	⬆️
#syntax	28.35% <ø> (ø)	⬆️

[erak]

I'd add this to https://solidity.readthedocs.io/en/v0.5.2/structure-of-a-contract.html#functions. Or is this explained somewhere else?

[ChrisChinchilla]

I feel like it's mentioned enough in docs/contracts/functions.rst (See the commits), but let me know what you think. When that's done I'll rebase @ekpyron

[axic]

This is the current functions sections:

Function Calls can happen internally or externally and have different levels of visibility towards other contracts. Functions accept parameters and return variables to pass parameters and values between them.

I'd say it would be reasonable to include something along the lines "Functions can also have special attributes (modifiers?), like payable [link here]"

[ChrisChinchilla]

@axic In the section below this paragraph is a section about function modifiers. Thing is, in your suggestion you mention linking to a list of function modifiers, and I agree, there isn't really a list (in one place) of what's available. So I think creating one is a good idea, but perhaps out of scope of this PR?

[erak]

@ChrisChinchilla That sounds like a good idea. Let's create an issue for that and merge this one :)

[ekpyron]

Needs to be rebased.

[erak]

@ChrisChinchilla That sounds like a good idea. Let's create an issue for that and merge this one :)

[ChrisChinchilla]

@bit-shift And there was a merge conflict, will need that approval again, sorry!