Avoid punning [param enabled] in documentation

[timothyqiu]

If [param enabled] the provided [param obstacle] affects avoidance using agents.

[param enabled] has to be kept in translation, so the pun will be lost because readers might not know what "enabled" means in English.

Currently, e.g. in Chinese, this sentence will be translated as if it's

If [param enabled] is [code]true[/code], the provided [param obstacle] affects avoidance using agents.

Weblate's complain about non-match BBCode has to be turned off manually for each entry. And either leaving a bare unmarked-up "true" in the description or translating "true" here feels wrong.

This PR changes If [param enabled] then XXX to If [param enabled] is [code]true[/code], then XXX, just like in other classrefs.

[rburing]

This makes the documentation clearer also in English.

[YuriSizov]

Thanks!