Harmonise and improve ⎕XML, ⎕JSON, and ⎕CSV mention of precision

**Page on which the enhancement should be made**
https://docs.dyalog.com/21.0/language-reference-guide/system-functions/csv
https://docs.dyalog.com/21.0/language-reference-guide/system-functions/json
https://docs.dyalog.com/21.0/language-reference-guide/system-functions/xml

**Describe the enhancement you would like**
For XML, don't claim it is like APL's format with `⎕PP←17`, as it isn't. For all of them, make it clear that the produced text can differ from APL's native formatting.

Review the CSV, JSON, and XML system-function pages linked in the issue, focusing on their statements about precision and native APL formatting. Harmonize the wording so each page makes clear that produced text may differ from APL's native formatting, and remove the inaccurate XML comparison; done means all three pages describe this consistently.

Written by the indexing model from the issue text.