Update titles and descriptions for models

[lboeman]

Update models.py with titles and descriptions where needed using the text proposed by Mark.

The title property of each object is used on the dashboard to label the input, and is also visible in the API documentation.
The description property is used as the "?" popup text.

I've started here by updating the PVSystem fields as an example. I've omitted the "Enter" from the beginning of descriptions as they seemed they would be out of place in the API documentation.

[codecov]

Codecov Report

Merging #202 (cdc1322) into main (b6dbc9b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #202   +/-   ##
=======================================
  Coverage   87.75%   87.75%           
=======================================
  Files          85       85           
  Lines        3651     3651           
  Branches      331      331           
=======================================
  Hits         3204     3204           
  Misses        382      382           
  Partials       65       65           

Flag	Coverage Δ
python	99.19% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api/solarperformanceinsight_api/models.py	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b6dbc9b...cdc1322. Read the comment docs.