Size Estimator wrongly calculate Disk space in MB [LUCENE-8329]

The size estimator dev tool ( dev-tools/size-estimator-lucene-solr.xls )currently :
- Wrongly calculates disk size in MB ( showing GB)
- Doesn't specify clearly that the space needed by the optimize is FREE space
- Avg. Document Size (KB) when they are more correctly Avg. Document Field Size (KB)

Scope of this issue is just to fix these small mistakes.
Out of scope is any improvement to the tool ( potentially separate Jira issues will follow)

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Migrated from [LUCENE-8329](https://issues.apache.org/jira/browse/LUCENE-8329) by Alessandro Benedetti (@alessandrobenedetti), updated Jan 24 2020
Attachments: [LUCENE-8329.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8329/LUCENE-8329.patch)
Pull requests: https://github.com/apache/lucene-solr/pull/381

Open dev-tools/size-estimator-lucene-solr.xls and compare it with the attached LUCENE-8329.patch. Verify the three requested corrections: disk space must display MB, optimize space must be identified as free space, and the label must say Avg. Document Field Size (KB). The work is done when the spreadsheet reflects these corrections without broader improvements.

Written by the indexing model from the issue text.