chore(pipelines): clarify when not to use the modern synthesis flag (#18873)

Closes #18572.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

packages/@aws-cdk/pipelines/README.md

@@ -944,22 +944,30 @@ or future deployments to this environment will fail. If you want to upgrade
 the bootstrap stack to a newer version, do that by updating it in-place.
 
 > This library requires the *modern* bootstrapping stack which has
-> been updated specifically to support cross-account continuous delivery. Starting,
-> in CDK v2 this new bootstrapping stack will become the default, but for now it is still
-> opt-in.
+> been updated specifically to support cross-account continuous delivery.
 >
-> The commands below assume you are running `cdk bootstrap` in a directory
-> where `cdk.json` contains the `"@aws-cdk/core:newStyleStackSynthesis": true`
-> setting in its context, which will switch to the new bootstrapping stack
-> automatically.
+> If you are using CDKv2, you do not need to do anything else. Modern
+> bootstrapping and modern stack synthesis (also known as "default stack
+> synthesis") is the default.
 >
-> If run from another directory, be sure to run the bootstrap command with
-> the environment variable `CDK_NEW_BOOTSTRAP=1` set.
+> If you are using CDKv1, you need to opt in to modern bootstrapping and
+> modern stack synthesis using a feature flag. Make sure `cdk.json` includes:
+>
+> ```json
+> {
+>   "context": {
+>     "@aws-cdk/core:newStyleStackSynthesis": true
+>   }
+> }
+> ```
+>
+> And be sure to run `cdk bootstrap` in the same directory as the `cdk.json`
+> file.
 
 To bootstrap an environment for provisioning the pipeline:
 
 ```console
-$ env CDK_NEW_BOOTSTRAP=1 npx cdk bootstrap \
+$ npx cdk bootstrap \
     [--profile admin-profile-1] \
     --cloudformation-execution-policies arn:aws:iam::aws:policy/AdministratorAccess \
     aws://111111111111/us-east-1
@@ -969,7 +977,7 @@ To bootstrap a different environment for deploying CDK applications into using
 a pipeline in account `111111111111`:
 
 ```console
-$ env CDK_NEW_BOOTSTRAP=1 npx cdk bootstrap \
+$ npx cdk bootstrap \
     [--profile admin-profile-2] \
     --cloudformation-execution-policies arn:aws:iam::aws:policy/AdministratorAccess \
     --trust 11111111111 \
@@ -980,7 +988,7 @@ If you only want to trust an account to do lookups (e.g, when your CDK applicati
 `Vpc.fromLookup()` call), use the option `--trust-for-lookup`:
 
 ```console
-$ env CDK_NEW_BOOTSTRAP=1 npx cdk bootstrap \
+$ npx cdk bootstrap \
     [--profile admin-profile-2] \
     --cloudformation-execution-policies arn:aws:iam::aws:policy/AdministratorAccess \
     --trust-for-lookup 11111111111 \