Add information about metrics

[snk4tr]

Is your feature request related to a problem? Please describe.
All metrics have important characteristics such as:

• Possible input values (e.g. [0, 1] or [1, 255])
• Outputs range (e.g [-1, 1] or [0, +inf])
• Symmetry:f(x, y) = f(y, x)
• Additional information that we gained while working with the library (e.g. VSI has an actual dynamic range of [0.95, 1.0] instead of [0, 1]

Currently, no such information is added to the existing metrics.

Describe the solution you'd like

1. Add the required information in notes section of docstrings of all metrics in the library.
2. Move the notes section up in the readthedocs documentation so it is more visible to a user.

Additional context
Related and partially triggered by discussions on #315