fix(srv): add info on function concurrency

Author: SamyOubouaziz

pages/serverless-functions/concepts.mdx

@@ -39,6 +39,10 @@ Startup process steps are:
 
 [How to reduce cold starts](/faq/serverless-functions/#how-to-reduce-cold-start-of-serverless-functions)
 
+## Concurrency
+
+Concurrency defines the capacity of a resource to process several requests at the same time. A single instance of a function has a concurrency of `1` as it handles requests  sequentially, one by one, but a Serverless Function can have several instances running in parallel, depending on its [scaling](#scaling) configuration.
+
 ## Container Registry
 
 Container Registry is the place where your images of your Serverless Functions are stored before being deployed.

pages/serverless-functions/reference-content/functions-limitations.mdx

@@ -21,27 +21,29 @@ This section contains usage limits that apply when using Serverless Functions.
 | Functions                                           | Max number | 1000*            | Organization                |
 | Total function memory\**                            | Max size   | 600 GiB          | Organization                |
 | Custom domains                                      | Max number | 50               | Function                    |
-| Zip Size                                            | Max size   | 100 MiB          | Function                    |
-| Code Size                                           | Max size   | 500 MiB          | Function                    |
+| Zip size                                            | Max size   | 100 MiB          | Function                    |
+| Code size                                           | Max size   | 500 MiB          | Function                    |
 | Temporary disk size                                 | Max size   | 1024 MiB         | Function instance           |
 | Invocation rate                                     | Max number | 1000 per second  | Function                    |
-| Concurrency                                         | Max        | 1                | Function instance           |
-| Scaling (Number of simultaneous Function Instances) | Max        | 50               | Function                    |
-| Environment Variables                               | Max number | 100              | Function + Namespace        |
-| Environment Variables                               | Max size   | 1000 chars       | Environment Variable        |
-| Secret Environment Variables                        | Max number | 100              | Function + Namespace        |
-| Secret Environment Variables                        | Max size   | 65536 bytes      | Secret Environment Variable |
+| Concurrency (number of simultaneous requests)\***   | Max        | 1                | Function instance           |
+| Scaling (number of simultaneous function instances) | Max        | 50               | Function                    |
+| Environment variables                               | Max number | 100              | Function + Namespace        |
+| Environment variables                               | Max size   | 1000 chars       | Environment variable        |
+| Secret environment variables                        | Max number | 100              | Function + Namespace        |
+| Secret environment variables                        | Max size   | 65536 bytes      | Secret environment variable |
 | Time before scale to zero                           | Time       | 15 minutes       | Instance                    |
 | Time before scale down                              | Time       | 30 seconds       | Instance                    |
-| Max HTTP request duration\***                       | Max        | 15 minutes       | Request                     |
+| Max HTTP request duration\****                      | Max        | 15 minutes      | Request                     |
 | Payload                                             | Max size   | 6 MiB            | Request                     |
 | Logs                                                | Logs       | 30000 per minute | Project                     |
 
 \* Lower limits may apply before account verification. [Contact our support team](https://console.scaleway.com/support) if you have any questions.
 
 \** Total function memory is the sum of the memory allocated to all your functions at their maximum Scale.
 
-\*** Maximum duration of a single HTTP request before this request is timed out. Use [Serverless Jobs](/serverless-jobs/) for tasks up to 24h.
+\*** Each instance of a function can handle a single request at a time, but a function can have several instances running in parallel.
+
+\**** Maximum duration of a single HTTP request before this request is timed out. Use [Serverless Jobs](/serverless-jobs/) for tasks up to 24h.
 
 These limits are enforced as [Organization quotas](/organizations-and-projects/additional-content/organization-quotas/#serverless-functions).