Improve 'Modeling with BIS' doc page

[ColinKerr]

The following concerns were raised:

Page https://www.itwinjs.org/bis/guide/intro/modeling-with-bis - "Modeling with BIS"

• "BIS conceives of Objects as composed of multiple Entities,". "Conceives" -> "thinks"
• "BIS escapes from infinite regression by defining a special RepositoryModel that is not required to sub-model some other Element.". "Regression" -> "model models element loop"
• "There can be many different kinds of Relationships among Elements within a Model or spanning Models.". "Spanning" -> "multiple related"

The problem is larger than the individual word choices and the area highlighted by these comments needs larger rewrites