docs(java): Describe request context usage

[dustinbyrne]

Changes

Adds Java SDK documentation for request context support:

• Shows how to build request context from X-PostHog-Distinct-Id and X-PostHog-Session-Id headers
• Documents attaching request metadata such as $current_url, $request_method, $request_path, $user_agent, and $ip
• Shows using scoped context for capture, captureException, and evaluateFlags
• Notes how to ignore tracing headers, the client-controlled nature of these headers, and ThreadLocal propagation limits

Checklist

• I've read the docs and/or content style guides.
• Words are spelled using American English
• Use relative URLs for internal links
• I've checked the pages added or changed in the Vercel preview build
• If I moved a page, I added a redirect in vercel.json

[github-actions]

Deploy preview

Status	Details	Updated (UTC)
🟢 Ready	View preview	May 28, 2026 02:15PM

[github-actions]

Vale prose linter → found 0 errors, 13 warnings, 0 suggestions in your markdown

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contents/docs/libraries/java/index.mdx — 0 errors, 13 warnings, 0 suggestions

Line	Severity	Message	Rule
6:18	warning	Use 'GitHub' instead of 'github'.	Vale.Terms
6:37	warning	Use 'PostHog' instead of 'posthog'.	Vale.Terms
6:45	warning	Use 'Android' instead of 'android'.	Vale.Terms
122:192	warning	Capitalize 'Session Replay' for PostHog's product. Use 'session replay' for the general industry concept.	PostHogBase.ProductNames
122:208	warning	Capitalize 'Error Tracking' for PostHog's product. Use 'error tracking' for the general industry concept.	PostHogBase.ProductNames
159:219	warning	'personless' is a possible misspelling.	PostHogBase.Spelling
175:4	warning	'Feature flags' heading should be in sentence case, and product names should be capitalized.	PostHogBase.SentenceCase
175:4	warning	Capitalize 'Feature Flags' for PostHog's product. Use 'Feature flags' for the general industry concept.	PostHogBase.ProductNames
323:8	warning	Capitalize 'Experiments' for PostHog's product. Use 'experiments' for the general industry concept.	PostHogBase.ProductNames
323:55	warning	Capitalize 'Feature Flags' for PostHog's product. Use 'feature flags' for the general industry concept.	PostHogBase.ProductNames
323:128	warning	Capitalize 'Feature Flags' for PostHog's product. Use 'feature flags' for the general industry concept.	PostHogBase.ProductNames
335:28	warning	Capitalize 'Experiments' for PostHog's product. Use 'experiments' for the general industry concept.	PostHogBase.ProductNames
335:54	warning	Capitalize 'Feature Flags' for PostHog's product. Use 'feature flags' for the general industry concept.	PostHogBase.ProductNames