[Maps] fix scaling and term join in product help popover width

[nreese]

elastic/eui#5895 converted EuiText from CSS to Emotion. This change removed max-width: '36em' style from EuiText.

Scaling form and Term join in product help relied on this max-width setting to determine popover width. Without these, popovers look terrible as seen in image below

Chatted with EUI team about problem and they tried to solve problem in EUI, elastic/eui#6146. However, EUI determined the old behavior was a bug and Maps should set max-width

PR restores original popover look by setting max-width to what was previously set in EuiText CSS.

To test

1. install sample web logs data set
2. create new map
3. click add layer
4. add documents layer, select web logs index pattern, click add layer
5. open popover for scaling and term joins. Verify width is not over sized.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 0ed9dd6

Failed CI Steps

• FTR Configs #36

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
maps	2.6MB	2.6MB	+32.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[nickpeihl]

lgtm!

code review and tested in chrome