Add to Word choice section

[jpmckinney]

https://ocds-standard-development-handbook.readthedocs.io/en/latest/meta/style_guide.html#word-choice

Off the top of my head:

• "potential supplier" (vs "supplier", "tenderer")
• "buyer or procuring entity"
• "planning process" not "planning stage"

@JachymHercher Please help add more bullets, based on recent changes / discussions.