Update string field value size limit from 64KB to 1MB (#7054)

* Initial plan

* docs(line-protocol): update string field value size limit from 64KB to 1MB

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---------

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Co-authored-by: jstirnaman <212227+jstirnaman@users.noreply.github.com>

Author: Copilot

content/shared/v3-line-protocol.md

@@ -150,7 +150,11 @@ myTable fieldKey=12485903u
 ### String
 
 Plain text string.
-Length limit 64KB.
+
+| Element | Maximum length |
+|:--------|:---------------|
+| Table name, tag key, tag value, field key | 64KB |
+| Field value | 1MB |
 
 ##### String example