[8.9] [docs] Explain max payload size in RUM source map docs (elastic#165732) (elastic#166278)

# Backport

This will backport the following commits from `main` to `8.9`:
- [[docs] Explain max payload size in RUM source map docs
(elastic#165732)](elastic#165732)

<!--- Backport version: 8.9.7 -->

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Explain max payload size in RUM source map docs (elastic#165732)\n\n##
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Explain max payload size in RUM source map docs (elastic#165732)\n\n##
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Co-authored-by: Brandon Morelli <[email protected]>

Author: kibanamachine

docs/apm/api.asciidoc

@@ -503,6 +503,16 @@ The following APIs are available:
 * <<rum-sourcemap-get>>
 * <<rum-sourcemap-delete>>
 
+[float]
+[[limit-sourcemap-api]]
+==== Max payload size
+
+{kib}'s maximum payload size is 1mb.
+If you attempt to upload a source map that exceeds the max payload size, you will get a `413` error.
+
+Before uploading source maps that exceed this default, change the maximum payload size allowed by {kib}
+with the <<server-maxPayload,`server.maxPayload`>> variable.
+
 [float]
 [[use-sourcemap-api]]
 ==== How to use APM APIs