docs: Add warning about react errors and default ignores (#840)

Author: kamilogorek

src/collections/_documentation/platforms/javascript/default-integrations.md

@@ -27,6 +27,8 @@ _Import name: `Sentry.Integrations.InboundFilter`_
 
 This integration allows developers to ignore specific errors based on the type or message,
 as well as blacklist/whitelist urls which exception originates from.
+It ignores errors, which message starts with `Script error` or `Javascript error: Script error` by default.
+More on this in our ["What the heck is "Script error"?"](https://blog.sentry.io/2016/05/17/what-is-script-error) blog post.
 
 To configure it, use `ignoreErrors`, `blacklistUrls` and `whitelistUrls` SDK options directly.
 

src/collections/_documentation/platforms/javascript/index.md

@@ -793,6 +793,7 @@ This integration deduplicates certain events. The Sentry SDK enables this by def
 _Import name: `Sentry.Integrations.InboundFilter`_
 
 This integration allows developers to ignore specific errors based on the type or message, as well as blacklist/whitelist URLs that originate from the exception.
+It ignores errors, which message starts with `Script error` or `Javascript error: Script error` by default. More on this in our ["What the heck is "Script error"?"](https://blog.sentry.io/2016/05/17/what-is-script-error) blog post.
 
 To configure it, use `ignoreErrors`, `blacklistUrls`, and `whitelistUrls` SDK options directly.
 

src/collections/_documentation/platforms/javascript/react.md

@@ -8,6 +8,15 @@ On its own, `@sentry/browser` will report any uncaught exceptions triggered from
 
 If you’re using React 16 or above, Error Boundaries are an important tool for defining the behavior of your application in the face of errors. Be sure to send errors they catch to Sentry using `Sentry.captureException`, and optionally this is also a great opportunity to surface User Feedback
 
+{% capture __alert_content -%}
+One important thing to note about the behavior of error boundaries in development mode is that React will rethrow errors they catch. This will result in errors being reported twice to Sentry with the above setup, but this won’t occur in your production build.
+{%- endcapture -%}
+{%- include components/alert.html
+  title="Note"
+  content=__alert_content
+  level="info"
+%}
+
 ```jsx
 import * as Sentry from '@sentry/browser';