Refine explanation of individual reproduction number

[avallecam]

Clarified the definition and estimation of the individual reproduction number in the context of outbreak analysis.

• Please check if the PR fulfills these requirements

• I have read the CONTRIBUTING guidelines
• A new item has been added to NEWS.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Checks have been run locally and pass

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

  • Refine wording and explanation to reduce confusions detected in practical sessions due to material inconsistency.

• What is the current behavior? (You can also link to an open issue here)

  • The definition was not sync with the definitions in the terminology recap.

• What is the new behavior (if this is a feature change)?

  • It clarifies that we refer to secondary infections from infected individuals, and additionally link it with the R0 and its individual-level variability.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

  • No.

• Other information:

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