"Upgrade" Gulp Task: Add note about Bootstrap 3's box model changes

[blzaugg]

Near this console output:

----------------------------------------------------------------
 Bootstrap Upgrade (2 to 3)
----------------------------------------------------------------

File: src/css/custom.css
    Line 207: Padding no longer affects width or height, you may need to change your rule (lines 207-215)
    Line 250: Padding no longer affects width or height, you may need to change your rule (lines 250-262)
    Line 264: Padding no longer affects width or height, you may need to change your rule (lines 264-281)
    Line 283: Padding no longer affects width or height, you may need to change your rule (lines 283-296)
    Line 369: Padding no longer affects width or height, you may need to change your rule (lines 369-378)
    Line 666: Padding no longer affects width or height, you may need to change your rule (lines 666-681)
    Line 675: You would change width from "150px" to "170px"

Add some additional explanation WHY padding and width/height will cause issues.

Something like:

Note: Because Liferay Portal 7.0 uses Bootstrap 3, the default box model has been changed to box-sizing: border-box. So if you were using width or height, and padding together on an element, you may need to make changes, or those elements may have unexpected sizes.

[jbalsas]

/cc @mwilliams2014, just in case this is still missing in our docs, since I'm closing over here. Code is moving to liferay-themes-sdk

[mwilliams2014]

Documented here: https://dev.liferay.com/en/develop/tutorials/-/knowledge_base/7-0/upgrading-themes#updating-css-rules