Enhance doc and alert text (based on v4.6dev)

[grumpyinca]

While based on a master in the 4.6dev timeframe, the doc changes contemplated at the time that this issue is opened are not strongly tied to the 4.6 release (or any specific release). Thus, the changes created in this branch may ultimately become part of a different release.

/docs/Help/DesignTypes/Spring/{type}/alerts.md
expand discussion of:

• %_Avail_Deflect @ 2 > 80% (alerts.html#PC_Avail_Deflect2_GT_80)
• %_Avail_Deflect @ 1 < 20% (alerts.html#PC_Avail_Deflect1_LT_20)

/docs/Help/alerts.md

[grumpyinca]

Create a new "Guidelines" entry under docs/design. Use it to centralize:

• version number guidelines (existing)
• image placement guidelines (existing)
• tooltip image guidelines (see: Add images to variable name tooltips #897)

[grumpyinca]

Add a statement to the GitHub README along the lines of:

If you are developing software that utilizes any of this material, please contact us. We would like to work with you.
Specifically, we may be able to integrate pull requests such that your work can more easily take advantage of our future enhancements.