[DOCS] Clarify behavior of SummarizingConversationManager: only the oldest `summary_ratio` messages are summarized

[grandmasteri]

Documentation Section

https://strandsagents.com/latest/documentation/docs/user-guide/concepts/agents/conversation-management/#summarizingconversationmanager

Current State

The SummarizingConversationManager implements intelligent conversation context management by summarizing older messages instead of simply discarding them. This approach preserves important information while staying within context limits.

• summary_ratio (float, default: 0.3): Percentage of messages to summarize when reducing context (clamped between 0.1 and 0.8)

Suggested Improvement

The `SummarizingConversationManager` implements intelligent conversation context management by summarizing older messages instead of simply discarding them. This approach preserves important information while staying within context limits.
-- - `summary_ratio` (float, default: 0.3): Percentage of messages to summarize when reducing context (clamped between 0.1 and 0.8)
+- - `summary_ratio` (float, default: 0.3): Percentage of oldest messages to summarize and replace with their summary when reducing context (clamped between 0.1 and 0.8)

Rationale

The current documentation doesn't make clear how exactly the summary_ratio parameter is used. Based on the source code, the suggested improvement describes how it's used.

Additional Context

No response