Fixes in Solr docs

[HenningTimm]

What this PR does / why we need it: This PR introduces some minor fixes and (hopefully) improvements to the Solr documentation. Most notably it reframes a link to a resolved issue associated with Solr reindexing. While indexing is still a major endeavor, the previous wording sounded to me like the Issue was still in active development. This PR keeps the important information in place (that reindexing takes some time). However, it changes the wording such that the link serves more as documentation than an issues that is actively worked on.

Further, this PR add some minor style fixes:

• Consistency of Solr casing (Solr with a capital S seems to be the right version.)
• Minor typographic fixes
• Extend section on reindexing to explain what a "completely clean index" means

Is there a release notes update needed for this change?: No