[21054] Make explicit the need to keep alive the flow controller descriptor name

[Mario-DL]

Description

#793 Introduced an ABI break changing const char* to std:.string in the flow controller descriptor name. This PR warns the user in the sample snippets that the name must be kept alive during participants lifetime.

Important This PR has to be cherry picked on the following backports:

• [21136] Add XML configuration for FlowControllerDescriptor to 2.x (backport #793) #802
• [21136] Add XML configuration for FlowControllerDescriptor to 2.x (backport #793) #803
• [21136] Add XML configuration for FlowControllerDescriptor to 2.x (backport #793) #804

Contributor Checklist

• Commit messages follow the project guidelines.
• Code snippets related to the added documentation have been provided.
• Documentation tests pass locally.
• NO (see PR description) Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• CI passes without warnings or errors.

[JesusPoderoso]

@richiprosima please test this

[EduPonz]

@Mario-DL please remember to cherry-pick this onto the other backports