[BD-5343] Add filter definitions for channel-specific reachable users

[Copilot]

Why are you making this change? (required)

Clarifies which filters Braze applies when calculating reachable users for each messaging channel.

Related PRs, issues, or features (optional)

• BD-5343

Feature release date (optional)

• N/A

Changes

Added filter specifications to two sections in measuring_segment_size.md:

• Email: Email Available = True
• iOS Push: Foreground Push Enabled iOS = True
• Android Push: Foreground Push Enabled Android = True
• Web Push: Foreground Push Enabled Web = True

Appears in both "Total reachable users calculation" and "Reachable users by channel" sections to provide consistent context wherever channel reachability is explained.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>[BD-5343]:Provide definition of what filters are applied for each channel</issue_title>
<issue_description>Description: Specify the filters we use for channel-specific reachable users:
Email: Email Available = True
iOS Push: Foreground Push Enabled iOS = True
Android Push: Foreground Push Enabled Android = True
Web Push: Foreground Push Enabled web = True

Request Url: https://www.braze.com/docs/user_guide/engagement_tools/segments/measuring_segment_size/#reachable-users-by-channel
JIRA: BD-5343</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [BD-5343]:Provide definition of what filters are applied for each channel #11027

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Fixes #11027

[Copilot]

Pull request overview

This PR clarifies which filters Braze applies when calculating reachable users for each messaging channel by adding a new table that explicitly defines the filter criteria for Email, iOS Push, Android Push, Web Push, and SMS channels.

• Adds a new "Applied filters for channel-specific reachable users" section with a table detailing specific filters for each channel
• Minor punctuation and grammar adjustments to existing text about reachable user requirements