Add better documention of PATHSolver.solve_mcp

[odow]

Part of #72

I had never really intended for people to use the solve_mcp directly, so it wasn't well documented. However, it seems like people are, so we should improve things.

cc @sdirkse67

[codecov[bot]]

Codecov Report

Base: 91.32% // Head: 91.32% // No change to project coverage :thumbsup:

Coverage data is based on head (91216a8) compared to base (2087cc0). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #73 +/- ## ======================================= Coverage 91.32% 91.32% ======================================= Files 3 3 Lines 346 346 ======================================= Hits 316 316 Misses 30 30 ``` | [Impacted Files](https://codecov.io/gh/chkwon/PATHSolver.jl/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Changhyun+Kwon) | Coverage Δ | | |---|---|---| | [src/C\_API.jl](https://codecov.io/gh/chkwon/PATHSolver.jl/pull/73/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Changhyun+Kwon#diff-c3JjL0NfQVBJLmps) | `91.04% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Changhyun+Kwon). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Changhyun+Kwon)

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