Expand description of tiss_cult_growth_med slot

[pkalita-lbl]

Currently the description of the tiss_cult_growth_med slot is a bit vague, and it is also not well aligned with the example shown. The description simply says:

Description of plant tissue culture growth media used

And the one example is a URL:

https://link.springer.com/content/pdf/10.1007/BF02796489.pdf

This caused confusion for a Field Notes user (microbiomedata/nmdc-field-notes#188) because they weren't sure what was really being asked of them.

I would suggest that:

1. The description should be updated to indicate that either a textual description or a URL where a description of the method can be found.
2. A second example should be added to illustrate an actual description (as opposed to a URL)