CHG: Improve signature and doc of synthdata routines

[tensionhead]

Changes Summary

• now every spy.synthdata function as a documented nTrials signature entry
• to produce only numpy arrays, use nTrials=None

Reviewer Checklist

• [ ] Are testing routines present?
• [ ] Do objects in the global package namespace perform proper parsing of their input?
• [ ] Are all docstrings complete and accurate?
• [ ] Is the CHANGELOG.md up to date?

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.30 :warning:

Comparison is base (c16a5d2) 69.24% compared to head (ad755d6) 68.95%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #502 +/- ## ========================================== - Coverage 69.24% 68.95% -0.30% ========================================== Files 87 87 Lines 10158 10162 +4 Branches 2073 2073 ========================================== - Hits 7034 7007 -27 - Misses 2598 2617 +19 - Partials 526 538 +12 ``` | [Impacted Files](https://app.codecov.io/gh/esi-neuroscience/syncopy/pull/502?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=esi-neuroscience) | Coverage Δ | | |---|---|---| | [syncopy/synthdata/analog.py](https://app.codecov.io/gh/esi-neuroscience/syncopy/pull/502?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=esi-neuroscience#diff-c3luY29weS9zeW50aGRhdGEvYW5hbG9nLnB5) | `75.80% <100.00%> (+0.39%)` | :arrow_up: | | [syncopy/synthdata/utils.py](https://app.codecov.io/gh/esi-neuroscience/syncopy/pull/502?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=esi-neuroscience#diff-c3luY29weS9zeW50aGRhdGEvdXRpbHMucHk=) | `100.00% <100.00%> (ø)` | | | [syncopy/tests/backend/test\_conn.py](https://app.codecov.io/gh/esi-neuroscience/syncopy/pull/502?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=esi-neuroscience#diff-c3luY29weS90ZXN0cy9iYWNrZW5kL3Rlc3RfY29ubi5weQ==) | `100.00% <100.00%> (ø)` | | ... and [8 files with indirect coverage changes](https://app.codecov.io/gh/esi-neuroscience/syncopy/pull/502/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=esi-neuroscience)

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